Merge pull request #7641 from layus/coerce-strings-fixups

Coerce strings fixups

30 files changed, 2289 insertions, 911 deletions

diff --git a/doc/manual/src/release-notes/rl-2.13.md b/doc/manual/src/release-notes/rl-2.13.md
index 2b79620be..2ebf19f60 100644
--- a/doc/manual/src/release-notes/rl-2.13.md
+++ b/doc/manual/src/release-notes/rl-2.13.md
@@ -18,6 +18,10 @@
 * Instead of "antiquotation", the more common term [string interpolation](../language/string-interpolation.md) is now used consistently.
   Historical release notes were not changed.
 
+* Error traces have been reworked to provide detailed explanations and more
+  accurate error locations. A short excerpt of the trace is now shown by
+  default when an error occurs.
+
 * Allow explicitly selecting outputs in a store derivation installable, just like we can do with other sorts of installables.
   For example,
   ```shell-session
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 60d6e9dc0..5090ea6d2 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -554,7 +554,7 @@ ref<eval_cache::EvalCache> openEvalCache(
             auto vFlake = state.allocValue();
             flake::callFlake(state, *lockedFlake, *vFlake);
 
-            state.forceAttrs(*vFlake, noPos);
+            state.forceAttrs(*vFlake, noPos, "while parsing cached flake data");
 
             auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
             assert(aOutputs);
@@ -618,7 +618,7 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
 
         else if (v.type() == nString) {
             PathSet context;
-            auto s = state->forceString(v, context, noPos);
+            auto s = state->forceString(v, context, noPos, fmt("while evaluating the flake output attribute '%s'", attrPath));
             auto storePath = state->store->maybeParseStorePath(s);
             if (storePath && context.count(std::string(s))) {
                 return {{
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index b7f691808..4158439b6 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -397,7 +397,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             Expr * e = parseString(expr);
             Value v;
             e->eval(*state, *env, v);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "while evaluating an attrset for the purpose of completion (this error should not be displayed; file an issue?)");
 
             for (auto & i : *v.attrs) {
                 std::string_view name = state->symbols[i.name];
@@ -590,7 +590,7 @@ bool NixRepl::processLine(std::string line)
         const auto [path, line] = [&] () -> std::pair<Path, uint32_t> {
             if (v.type() == nPath || v.type() == nString) {
                 PathSet context;
-                auto path = state->coerceToPath(noPos, v, context);
+                auto path = state->coerceToPath(noPos, v, context, "while evaluating the filename to edit");
                 return {path, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
@@ -839,7 +839,7 @@ void NixRepl::loadFiles()
 
 void NixRepl::addAttrsToScope(Value & attrs)
 {
-    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); });
+    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); }, "while evaluating an attribute set to be merged in the global scope");
     if (displ + attrs.attrs->size() >= envSize)
         throw Error("environment full; cannot add more variables");
 
@@ -944,7 +944,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
             Bindings::iterator i = v.attrs->find(state->sDrvPath);
             PathSet context;
             if (i != v.attrs->end())
-                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context));
+                str << state->store->printStorePath(state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the drvPath of a derivation"));
             else
                 str << "???";
             str << "»";
diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 94ab60f9a..7c0705091 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -118,7 +118,7 @@ std::pair<std::string, uint32_t> findPackageFilename(EvalState & state, Value &
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2);
+    auto pos = state.forceString(*v2, noPos, "while evaluating the 'meta.position' attribute of a derivation");
 
     auto colon = pos.rfind(':');
     if (colon == std::string::npos)
diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index f8c4275a1..1219b2471 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -385,7 +385,7 @@ Value & AttrCursor::getValue()
     if (!_value) {
         if (parent) {
             auto & vParent = parent->first->getValue();
-            root->state.forceAttrs(vParent, noPos);
+            root->state.forceAttrs(vParent, noPos, "while searching for an attribute");
             auto attr = vParent.attrs->get(parent->second);
             if (!attr)
                 throw Error("attribute '%s' is unexpectedly missing", getAttrPathStr());
@@ -571,14 +571,14 @@ std::string AttrCursor::getString()
                 debug("using cached string attribute '%s'", getAttrPathStr());
                 return s->first;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
+                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
+        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 
     return v.type() == nString ? v.string.s : v.path;
 }
@@ -613,7 +613,7 @@ string_t AttrCursor::getStringWithContext()
                     return *s;
                 }
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a string", getAttrPathStr()));
+                root->state.error("'%s' is not a string", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
@@ -624,7 +624,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path, {}};
     else
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a string but %s", getAttrPathStr(), showType(v.type())));
+        root->state.error("'%s' is not a string but %s", getAttrPathStr()).debugThrow<TypeError>();
 }
 
 bool AttrCursor::getBool()
@@ -637,14 +637,14 @@ bool AttrCursor::getBool()
                 debug("using cached Boolean attribute '%s'", getAttrPathStr());
                 return *b;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
+                root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nBool)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not a Boolean", getAttrPathStr()));
+        root->state.error("'%s' is not a Boolean", getAttrPathStr()).debugThrow<TypeError>();
 
     return v.boolean;
 }
@@ -696,7 +696,7 @@ std::vector<std::string> AttrCursor::getListOfStrings()
     std::vector<std::string> res;
 
     for (auto & elem : v.listItems())
-        res.push_back(std::string(root->state.forceStringNoCtx(*elem)));
+        res.push_back(std::string(root->state.forceStringNoCtx(*elem, noPos, "while evaluating an attribute for caching")));
 
     if (root->db)
         cachedValue = {root->db->setListOfStrings(getKey(), res), res};
@@ -714,14 +714,14 @@ std::vector<Symbol> AttrCursor::getAttrs()
                 debug("using cached attrset attribute '%s'", getAttrPathStr());
                 return *attrs;
             } else
-                root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
+                root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
         }
     }
 
     auto & v = forceValue();
 
     if (v.type() != nAttrs)
-        root->state.debugThrowLastTrace(TypeError("'%s' is not an attribute set", getAttrPathStr()));
+        root->state.error("'%s' is not an attribute set", getAttrPathStr()).debugThrow<TypeError>();
 
     std::vector<Symbol> attrs;
     for (auto & attr : *getValue().attrs)
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index f2f4ba725..f0da688db 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -103,33 +103,36 @@ void EvalState::forceValue(Value & v, Callable getPos)
     else if (v.isApp())
         callFunction(*v.app.left, *v.app.right, v, noPos);
     else if (v.isBlackhole())
-        throwEvalError(getPos(), "infinite recursion encountered");
+        error("infinite recursion encountered").atPos(getPos()).template debugThrow<EvalError>();
 }
 
 
 [[gnu::always_inline]]
-inline void EvalState::forceAttrs(Value & v, const PosIdx pos)
+inline void EvalState::forceAttrs(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceAttrs(v, [&]() { return pos; });
+    forceAttrs(v, [&]() { return pos; }, errorCtx);
 }
 
 
 template <typename Callable>
 [[gnu::always_inline]]
-inline void EvalState::forceAttrs(Value & v, Callable getPos)
+inline void EvalState::forceAttrs(Value & v, Callable getPos, std::string_view errorCtx)
 {
-    forceValue(v, getPos);
-    if (v.type() != nAttrs)
-        throwTypeError(getPos(), "value is %1% while a set was expected", v);
+    forceValue(v, noPos);
+    if (v.type() != nAttrs) {
+        PosIdx pos = getPos();
+        error("value is %1% while a set was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    }
 }
 
 
 [[gnu::always_inline]]
-inline void EvalState::forceList(Value & v, const PosIdx pos)
+inline void EvalState::forceList(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (!v.isList())
-        throwTypeError(pos, "value is %1% while a list was expected", v);
+    forceValue(v, noPos);
+    if (!v.isList()) {
+        error("value is %1% while a list was expected", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    }
 }
 
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 9bc20a502..1828b8c2e 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -11,7 +11,9 @@
 
 #include <algorithm>
 #include <chrono>
+#include <iostream>
 #include <cstring>
+#include <optional>
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
@@ -318,7 +320,7 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     } else {
         Value nameValue;
         name.expr->eval(state, env, nameValue);
-        state.forceStringNoCtx(nameValue);
+        state.forceStringNoCtx(nameValue, noPos, "while evaluating an attribute name");
         return state.symbols.create(nameValue.string.s);
     }
 }
@@ -414,6 +416,44 @@ static Strings parseNixPath(const std::string & s)
     return res;
 }
 
+ErrorBuilder & ErrorBuilder::atPos(PosIdx pos)
+{
+    info.errPos = state.positions[pos];
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withTrace(PosIdx pos, const std::string_view text)
+{
+    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = false });
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withFrameTrace(PosIdx pos, const std::string_view text)
+{
+    info.traces.push_front(Trace{ .pos = state.positions[pos], .hint = hintformat(std::string(text)), .frame = true });
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withSuggestions(Suggestions & s)
+{
+    info.suggestions = s;
+    return *this;
+}
+
+ErrorBuilder & ErrorBuilder::withFrame(const Env & env, const Expr & expr)
+{
+    // NOTE: This is abusing side-effects.
+    // TODO: check compatibility with nested debugger calls.
+    state.debugTraces.push_front(DebugTrace {
+        .pos = nullptr,
+        .expr = expr,
+        .env = env,
+        .hint = hintformat("Fake frame for debugging purposes"),
+        .isError = true
+    });
+    return *this;
+}
+
 
 EvalState::EvalState(
     const Strings & _searchPath,
@@ -646,25 +686,7 @@ void EvalState::addConstant(const std::string & name, Value * v)
 Value * EvalState::addPrimOp(const std::string & name,
     size_t arity, PrimOpFun primOp)
 {
-    auto name2 = name.substr(0, 2) == "__" ? name.substr(2) : name;
-    auto sym = symbols.create(name2);
-
-    /* Hack to make constants lazy: turn them into a application of
-       the primop to a dummy value. */
-    if (arity == 0) {
-        auto vPrimOp = allocValue();
-        vPrimOp->mkPrimOp(new PrimOp { .fun = primOp, .arity = 1, .name = name2 });
-        Value v;
-        v.mkApp(vPrimOp, vPrimOp);
-        return addConstant(name, v);
-    }
-
-    Value * v = allocValue();
-    v->mkPrimOp(new PrimOp { .fun = primOp, .arity = arity, .name = name2 });
-    staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
-    baseEnv.values[baseEnvDispl++] = v;
-    baseEnv.values[0]->attrs->push_back(Attr(sym, v));
-    return v;
+    return addPrimOp(PrimOp { .fun = primOp, .arity = arity, .name = name });
 }
 
 
@@ -842,176 +864,14 @@ void EvalState::runDebugRepl(const Error * error, const Env & env, const Expr &
     }
 }
 
-/* Every "format" object (even temporary) takes up a few hundred bytes
-   of stack space, which is a real killer in the recursive
-   evaluator.  So here are some helper functions for throwing
-   exceptions. */
-void EvalState::throwEvalError(const PosIdx pos, const char * s, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const char * s, const std::string & s2)
-{
-    debugThrowLastTrace(EvalError(s, s2));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const std::string & s2, Env & env, Expr & expr)
-{
-    debugThrow(EvalError(ErrorInfo{
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos],
-        .suggestions = suggestions,
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s, s2),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const char * s, const std::string & s2,
-    const std::string & s3)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[noPos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3)
-{
-    debugThrowLastTrace(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-    const std::string & s3, Env & env, Expr & expr)
-{
-    debugThrow(EvalError({
-        .msg = hintfmt(s, s2, s3),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2, Env & env, Expr & expr)
-{
-    // p1 is where the error occurred; p2 is a position mentioned in the message.
-    debugThrow(EvalError({
-        .msg = hintfmt(s, symbols[sym], positions[p2]),
-        .errPos = positions[p1]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v)
-{
-    debugThrowLastTrace(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const Value & v, Env & env, Expr & expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s)
-{
-    debugThrowLastTrace(TypeError({
-        .msg = hintfmt(s),
-        .errPos = positions[pos]
-    }));
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun,
-    const Symbol s2, Env & env, Expr &expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s,
-    const ExprLambda & fun, const Symbol s2, Env & env, Expr &expr)
-{
-    debugThrow(TypeError(ErrorInfo {
-        .msg = hintfmt(s, fun.showNamePos(*this), symbols[s2]),
-        .errPos = positions[pos],
-        .suggestions = suggestions,
-    }), env, expr);
-}
-
-void EvalState::throwTypeError(const char * s, const Value & v, Env & env, Expr &expr)
-{
-    debugThrow(TypeError({
-        .msg = hintfmt(s, showType(v)),
-        .errPos = positions[expr.getPos()],
-    }), env, expr);
-}
-
-void EvalState::throwAssertionError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(AssertionError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(UndefinedVarError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
-void EvalState::throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1, Env & env, Expr &expr)
-{
-    debugThrow(MissingArgumentError({
-        .msg = hintfmt(s, s1),
-        .errPos = positions[pos]
-    }), env, expr);
-}
-
 void EvalState::addErrorTrace(Error & e, const char * s, const std::string & s2) const
 {
     e.addTrace(nullptr, s, s2);
 }
 
-void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const
+void EvalState::addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame) const
 {
-    e.addTrace(positions[pos], s, s2);
+    e.addTrace(positions[pos], hintfmt(s, s2), frame);
 }
 
 static std::unique_ptr<DebugTraceStacker> makeDebugTraceStacker(
@@ -1088,7 +948,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
         if (env->type == Env::HasWithExpr) {
             if (noEval) return 0;
             Value * v = allocValue();
-            evalAttrs(*env->up, (Expr *) env->values[0], *v);
+            evalAttrs(*env->up, (Expr *) env->values[0], *v, noPos, "<borked>");
             env->values[0] = v;
             env->type = Env::HasWithAttrs;
         }
@@ -1098,7 +958,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
             return j->value;
         }
         if (!env->prevWith)
-            throwUndefinedVarError(var.pos, "undefined variable '%1%'", symbols[var.name], *env, const_cast<ExprVar&>(var));
+            error("undefined variable '%1%'", symbols[var.name]).atPos(var.pos).withFrame(*env, var).debugThrow<UndefinedVarError>();
         for (size_t l = env->prevWith; l; --l, env = env->up) ;
     }
 }
@@ -1248,7 +1108,7 @@ void EvalState::cacheFile(
         // computation.
         if (mustBeTrivial &&
             !(dynamic_cast<ExprAttrs *>(e)))
-            throw EvalError("file '%s' must be an attribute set", path);
+            error("file '%s' must be an attribute set", path).debugThrow<EvalError>();
         eval(e, v);
     } catch (Error & e) {
         addErrorTrace(e, "while evaluating the file '%1%':", resolvedPath);
@@ -1266,31 +1126,31 @@ void EvalState::eval(Expr * e, Value & v)
 }
 
 
-inline bool EvalState::evalBool(Env & env, Expr * e)
+inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos, std::string_view errorCtx)
 {
-    Value v;
-    e->eval(*this, env, v);
-    if (v.type() != nBool)
-        throwTypeError(noPos, "value is %1% while a Boolean was expected", v, env, *e);
-    return v.boolean;
-}
-
-
-inline bool EvalState::evalBool(Env & env, Expr * e, const PosIdx pos)
-{
-    Value v;
-    e->eval(*this, env, v);
-    if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v, env, *e);
-    return v.boolean;
+    try {
+        Value v;
+        e->eval(*this, env, v);
+        if (v.type() != nBool)
+            error("value is %1% while a Boolean was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
+        return v.boolean;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
+inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    e->eval(*this, env, v);
-    if (v.type() != nAttrs)
-        throwTypeError(noPos, "value is %1% while a set was expected", v, env, *e);
+    try {
+        e->eval(*this, env, v);
+        if (v.type() != nAttrs)
+            error("value is %1% while a set was expected", showType(v)).withFrame(env, *e).debugThrow<TypeError>();
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
@@ -1363,7 +1223,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
            Hence we need __overrides.) */
         if (hasOverrides) {
             Value * vOverrides = (*v.attrs)[overrides->second.displ].value;
-            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); });
+            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); }, "while evaluating the `__overrides` attribute");
             Bindings * newBnds = state.allocBindings(v.attrs->capacity() + vOverrides->attrs->size());
             for (auto & i : *v.attrs)
                 newBnds->push_back(i);
@@ -1391,11 +1251,11 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(nameVal, i.pos);
         if (nameVal.type() == nNull)
             continue;
-        state.forceStringNoCtx(nameVal);
+        state.forceStringNoCtx(nameVal, i.pos, "while evaluating the name of a dynamic attribute");
         auto nameSym = state.symbols.create(nameVal.string.s);
         Bindings::iterator j = v.attrs->find(nameSym);
         if (j != v.attrs->end())
-            state.throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, j->pos, env, *this);
+            state.error("dynamic attribute '%1%' already defined at %2%", state.symbols[nameSym], state.positions[j->pos]).atPos(i.pos).withFrame(env, *this).debugThrow<EvalError>();
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1492,15 +1352,14 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     return;
                 }
             } else {
-                state.forceAttrs(*vAttrs, pos);
+                state.forceAttrs(*vAttrs, pos, "while selecting an attribute");
                 if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end()) {
                     std::set<std::string> allAttrNames;
                     for (auto & attr : *vAttrs->attrs)
                         allAttrNames.insert(state.symbols[attr.name]);
-                    state.throwEvalError(
-                        pos,
-                        Suggestions::bestMatches(allAttrNames, state.symbols[name]),
-                        "attribute '%1%' missing", state.symbols[name], env, *this);
+                    auto suggestions = Suggestions::bestMatches(allAttrNames, state.symbols[name]);
+                    state.error("attribute '%1%' missing", state.symbols[name])
+                        .atPos(pos).withSuggestions(suggestions).withFrame(env, *this).debugThrow<EvalError>();
                 }
             }
             vAttrs = j->value;
@@ -1595,7 +1454,12 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             if (!lambda.hasFormals())
                 env2.values[displ++] = args[0];
             else {
-                forceAttrs(*args[0], pos);
+                try {
+                    forceAttrs(*args[0], lambda.pos, "while evaluating the value passed for the lambda argument");
+                } catch (Error & e) {
+                    if (pos) e.addTrace(positions[pos], "from call site");
+                    throw;
+                }
 
                 if (lambda.arg)
                     env2.values[displ++] = args[0];
@@ -1607,8 +1471,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 for (auto & i : lambda.formals->formals) {
                     auto j = args[0]->attrs->get(i.name);
                     if (!j) {
-                        if (!i.def) throwTypeError(pos, "%1% called without required argument '%2%'",
-                            lambda, i.name, *fun.lambda.env, lambda);
+                        if (!i.def) {
+                            error("function '%1%' called without required argument '%2%'",
+                                             (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
+                                             symbols[i.name])
+                                    .atPos(lambda.pos)
+                                    .withTrace(pos, "from call site")
+                                    .withFrame(*fun.lambda.env, lambda)
+                                    .debugThrow<TypeError>();
+                        }
                         env2.values[displ++] = i.def->maybeThunk(*this, env2);
                     } else {
                         attrsUsed++;
@@ -1626,11 +1497,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                             std::set<std::string> formalNames;
                             for (auto & formal : lambda.formals->formals)
                                 formalNames.insert(symbols[formal.name]);
-                            throwTypeError(
-                                pos,
-                                Suggestions::bestMatches(formalNames, symbols[i.name]),
-                                "%1% called with unexpected argument '%2%'",
-                                lambda, i.name, *fun.lambda.env, lambda);
+                            auto suggestions = Suggestions::bestMatches(formalNames, symbols[i.name]);
+                            error("function '%1%' called with unexpected argument '%2%'",
+                                             (lambda.name ? std::string(symbols[lambda.name]) : "anonymous lambda"),
+                                             symbols[i.name])
+                                .atPos(lambda.pos)
+                                .withTrace(pos, "from call site")
+                                .withSuggestions(suggestions)
+                                .withFrame(*fun.lambda.env, lambda)
+                                .debugThrow<TypeError>();
                         }
                     abort(); // can't happen
                 }
@@ -1653,11 +1528,15 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 lambda.body->eval(*this, env2, vCur);
             } catch (Error & e) {
                 if (loggerSettings.showTrace.get()) {
-                    addErrorTrace(e, lambda.pos, "while calling %s",
-                        (lambda.name
-                            ? concatStrings("'", symbols[lambda.name], "'")
-                            : "anonymous lambda"));
-                    addErrorTrace(e, pos, "while evaluating call site%s", "");
+                    addErrorTrace(
+                        e,
+                        lambda.pos,
+                        "while calling %s",
+                        lambda.name
+                        ? concatStrings("'", symbols[lambda.name], "'")
+                        : "anonymous lambda",
+                        true);
+                    if (pos) addErrorTrace(e, pos, "from call site%s", "", true);
                 }
                 throw;
             }
@@ -1676,9 +1555,17 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 return;
             } else {
                 /* We have all the arguments, so call the primop. */
+                auto name = vCur.primOp->name;
+
                 nrPrimOpCalls++;
-                if (countCalls) primOpCalls[vCur.primOp->name]++;
-                vCur.primOp->fun(*this, pos, args, vCur);
+                if (countCalls) primOpCalls[name]++;
+
+                try {
+                    vCur.primOp->fun(*this, noPos, args, vCur);
+                } catch (Error & e) {
+                    addErrorTrace(e, pos, "while calling the '%1%' builtin", name);
+                    throw;
+                }
 
                 nrArgs -= argsLeft;
                 args += argsLeft;
@@ -1713,9 +1600,20 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 for (size_t i = 0; i < argsLeft; ++i)
                     vArgs[argsDone + i] = args[i];
 
+                auto name = primOp->primOp->name;
                 nrPrimOpCalls++;
-                if (countCalls) primOpCalls[primOp->primOp->name]++;
-                primOp->primOp->fun(*this, pos, vArgs, vCur);
+                if (countCalls) primOpCalls[name]++;
+
+                try {
+                    // TODO:
+                    // 1. Unify this and above code. Heavily redundant.
+                    // 2. Create a fake env (arg1, arg2, etc.) and a fake expr (arg1: arg2: etc: builtins.name arg1 arg2 etc)
+                    //    so the debugger allows to inspect the wrong parameters passed to the builtin.
+                    primOp->primOp->fun(*this, noPos, vArgs, vCur);
+                } catch (Error & e) {
+                    addErrorTrace(e, pos, "while calling the '%1%' builtin", name);
+                    throw;
+                }
 
                 nrArgs -= argsLeft;
                 args += argsLeft;
@@ -1728,14 +1626,18 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                heap-allocate a copy and use that instead. */
             Value * args2[] = {allocValue(), args[0]};
             *args2[0] = vCur;
-            /* !!! Should we use the attr pos here? */
-            callFunction(*functor->value, 2, args2, vCur, pos);
+            try {
+                callFunction(*functor->value, 2, args2, vCur, functor->pos);
+            } catch (Error & e) {
+                e.addTrace(positions[pos], "while calling a functor (an attribute set with a '__functor' attribute)");
+                throw;
+            }
             nrArgs--;
             args++;
         }
 
         else
-            throwTypeError(pos, "attempt to call something which is not a function but %1%", vCur);
+            error("attempt to call something which is not a function but %1%", showType(vCur)).atPos(pos).debugThrow<TypeError>();
     }
 
     vRes = vCur;
@@ -1799,13 +1701,12 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
             if (j != args.end()) {
                 attrs.insert(*j);
             } else if (!i.def) {
-                throwMissingArgumentError(i.pos, R"(cannot evaluate a function that has an argument without a value ('%1%')
-
+                error(R"(cannot evaluate a function that has an argument without a value ('%1%')
 Nix attempted to evaluate a function as a top level expression; in
 this case it must have its arguments supplied either by default
 values, or passed explicitly with '--arg' or '--argstr'. See
-https://nixos.org/manual/nix/stable/language/constructs.html#functions.)", symbols[i.name],
-                *fun.lambda.env, *fun.lambda.fun);
+https://nixos.org/manual/nix/stable/language/constructs.html#functions.)", symbols[i.name])
+                    .atPos(i.pos).withFrame(*fun.lambda.env, *fun.lambda.fun).debugThrow<MissingArgumentError>();
             }
         }
     }
@@ -1828,16 +1729,17 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
 
 void ExprIf::eval(EvalState & state, Env & env, Value & v)
 {
-    (state.evalBool(env, cond, pos) ? then : else_)->eval(state, env, v);
+    // We cheat in the parser, and pass the position of the condition as the position of the if itself.
+    (state.evalBool(env, cond, pos, "while evaluating a branch condition") ? then : else_)->eval(state, env, v);
 }
 
 
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
-    if (!state.evalBool(env, cond, pos)) {
+    if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
         std::ostringstream out;
         cond->show(state.symbols, out);
-        state.throwAssertionError(pos, "assertion '%1%' failed", out.str(), env, *this);
+        state.error("assertion '%1%' failed", out.str()).atPos(pos).withFrame(env, *this).debugThrow<AssertionError>();
     }
     body->eval(state, env, v);
 }
@@ -1845,7 +1747,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 
 void ExprOpNot::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(!state.evalBool(env, e));
+    v.mkBool(!state.evalBool(env, e, noPos, "in the argument of the not operator")); // XXX: FIXME: !
 }
 
 
@@ -1853,7 +1755,7 @@ void ExprOpEq::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
-    v.mkBool(state.eqValues(v1, v2));
+    v.mkBool(state.eqValues(v1, v2, pos, "while testing two values for equality"));
 }
 
 
@@ -1861,33 +1763,33 @@ void ExprOpNEq::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
-    v.mkBool(!state.eqValues(v1, v2));
+    v.mkBool(!state.eqValues(v1, v2, pos, "while testing two values for inequality"));
 }
 
 
 void ExprOpAnd::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(state.evalBool(env, e1, pos) && state.evalBool(env, e2, pos));
+    v.mkBool(state.evalBool(env, e1, pos, "in the left operand of the AND (&&) operator") && state.evalBool(env, e2, pos, "in the right operand of the AND (&&) operator"));
 }
 
 
 void ExprOpOr::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
+    v.mkBool(state.evalBool(env, e1, pos, "in the left operand of the OR (||) operator") || state.evalBool(env, e2, pos, "in the right operand of the OR (||) operator"));
 }
 
 
 void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
 {
-    v.mkBool(!state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
+    v.mkBool(!state.evalBool(env, e1, pos, "in the left operand of the IMPL (->) operator") || state.evalBool(env, e2, pos, "in the right operand of the IMPL (->) operator"));
 }
 
 
 void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 {
     Value v1, v2;
-    state.evalAttrs(env, e1, v1);
-    state.evalAttrs(env, e2, v2);
+    state.evalAttrs(env, e1, v1, pos, "in the left operand of the update (//) operator");
+    state.evalAttrs(env, e2, v2, pos, "in the right operand of the update (//) operator");
 
     state.nrOpUpdates++;
 
@@ -1926,18 +1828,18 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
     Value * lists[2] = { &v1, &v2 };
-    state.concatLists(v, 2, lists, pos);
+    state.concatLists(v, 2, lists, pos, "while evaluating one of the elements to concatenate");
 }
 
 
-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx)
 {
     nrListConcats++;
 
     Value * nonEmpty = 0;
     size_t len = 0;
     for (size_t n = 0; n < nrLists; ++n) {
-        forceList(*lists[n], pos);
+        forceList(*lists[n], pos, errorCtx);
         auto l = lists[n]->listSize();
         len += l;
         if (l) nonEmpty = lists[n];
@@ -2014,20 +1916,22 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
                 nf = n;
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to an integer", showType(vTmp), env, *this);
+                state.error("cannot add %1% to an integer", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
         } else if (firstType == nFloat) {
             if (vTmp.type() == nInt) {
                 nf += vTmp.integer;
             } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint;
             } else
-                state.throwEvalError(i_pos, "cannot add %1% to a float", showType(vTmp), env, *this);
+                state.error("cannot add %1% to a float", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow<EvalError>();
         } else {
             if (s.empty()) s.reserve(es->size());
             /* skip canonization of first path, which would only be not
             canonized in the first place if it's coming from a ./${foo} type
             path */
-            auto part = state.coerceToString(i_pos, vTmp, context, false, firstType == nString, !first);
+            auto part = state.coerceToString(i_pos, vTmp, context,
+                                             "while evaluating a path segment",
+                                             false, firstType == nString, !first);
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -2041,7 +1945,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         v.mkFloat(nf);
     else if (firstType == nPath) {
         if (!context.empty())
-            state.throwEvalError(pos, "a string that refers to a store path cannot be appended to a path", env, *this);
+            state.error("a string that refers to a store path cannot be appended to a path").atPos(pos).withFrame(env, *this).debugThrow<EvalError>();
         v.mkPath(canonPath(str()));
     } else
         v.mkStringMove(c_str(), context);
@@ -2091,33 +1995,47 @@ void EvalState::forceValueDeep(Value & v)
 }
 
 
-NixInt EvalState::forceInt(Value & v, const PosIdx pos)
+NixInt EvalState::forceInt(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (v.type() != nInt)
-        throwTypeError(pos, "value is %1% while an integer was expected", v);
-
-    return v.integer;
+    try {
+        forceValue(v, pos);
+        if (v.type() != nInt)
+            error("value is %1% while an integer was expected", showType(v)).debugThrow<TypeError>();
+        return v.integer;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-NixFloat EvalState::forceFloat(Value & v, const PosIdx pos)
+NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (v.type() == nInt)
-        return v.integer;
-    else if (v.type() != nFloat)
-        throwTypeError(pos, "value is %1% while a float was expected", v);
-    return v.fpoint;
+    try {
+        forceValue(v, pos);
+        if (v.type() == nInt)
+            return v.integer;
+        else if (v.type() != nFloat)
+            error("value is %1% while a float was expected", showType(v)).debugThrow<TypeError>();
+        return v.fpoint;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-bool EvalState::forceBool(Value & v, const PosIdx pos)
+bool EvalState::forceBool(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v);
-    return v.boolean;
+    try {
+        forceValue(v, pos);
+        if (v.type() != nBool)
+            error("value is %1% while a Boolean was expected", showType(v)).debugThrow<TypeError>();
+        return v.boolean;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
@@ -2127,21 +2045,30 @@ bool EvalState::isFunctor(Value & fun)
 }
 
 
-void EvalState::forceFunction(Value & v, const PosIdx pos)
+void EvalState::forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (v.type() != nFunction && !isFunctor(v))
-        throwTypeError(pos, "value is %1% while a function was expected", v);
+    try {
+        forceValue(v, pos);
+        if (v.type() != nFunction && !isFunctor(v))
+            error("value is %1% while a function was expected", showType(v)).debugThrow<TypeError>();
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
+    }
 }
 
 
-std::string_view EvalState::forceString(Value & v, const PosIdx pos)
+std::string_view EvalState::forceString(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    forceValue(v, pos);
-    if (v.type() != nString) {
-        throwTypeError(pos, "value is %1% while a string was expected", v);
+    try {
+        forceValue(v, pos);
+        if (v.type() != nString)
+            error("value is %1% while a string was expected", showType(v)).debugThrow<TypeError>();
+        return v.string.s;
+    } catch (Error & e) {
+        e.addTrace(positions[pos], errorCtx);
+        throw;
     }
-    return v.string.s;
 }
 
 
@@ -2164,24 +2091,19 @@ NixStringContext Value::getContext(const Store & store)
 }
 
 
-std::string_view EvalState::forceString(Value & v, PathSet & context, const PosIdx pos)
+std::string_view EvalState::forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     copyContext(v, context);
     return s;
 }
 
 
-std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos)
+std::string_view EvalState::forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     if (v.string.context) {
-        if (pos)
-            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')",
-                v.string.s, v.string.context[0]);
-        else
-            throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
-                v.string.s, v.string.context[0]);
+        error("the string '%1%' is not allowed to refer to a store path (such as '%2%')", v.string.s, v.string.context[0]).withTrace(pos, errorCtx).debugThrow<EvalError>();
     }
     return s;
 }
@@ -2205,14 +2127,16 @@ std::optional<std::string> EvalState::tryAttrsToString(const PosIdx pos, Value &
     if (i != v.attrs->end()) {
         Value v1;
         callFunction(*i->value, v, v1, pos);
-        return coerceToString(pos, v1, context, coerceMore, copyToStore).toOwned();
+        return coerceToString(pos, v1, context,
+                "while evaluating the result of the `__toString` attribute",
+                coerceMore, copyToStore).toOwned();
     }
 
     return {};
 }
 
-BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet & context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath)
+BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &context,
+    std::string_view errorCtx, bool coerceMore, bool copyToStore, bool canonicalizePath)
 {
     forceValue(v, pos);
 
@@ -2235,13 +2159,23 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (maybeString)
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end())
-            throwTypeError(pos, "cannot coerce a set to a string");
-        return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
+        if (i == v.attrs->end()) {
+            error("cannot coerce %1% to a string", showType(v))
+                .withTrace(pos, errorCtx)
+                .debugThrow<TypeError>();
+        }
+        return coerceToString(pos, *i->value, context, errorCtx,
+                              coerceMore, copyToStore, canonicalizePath);
     }
 
-    if (v.type() == nExternal)
-        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+    if (v.type() == nExternal) {
+        try {
+            return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+        } catch (Error & e) {
+            e.addTrace(nullptr, errorCtx);
+            throw;
+        }
+    }
 
     if (coerceMore) {
         /* Note that `false' is represented as an empty string for
@@ -2255,7 +2189,14 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         if (v.isList()) {
             std::string result;
             for (auto [n, v2] : enumerate(v.listItems())) {
-                result += *coerceToString(pos, *v2, context, coerceMore, copyToStore);
+                try {
+                    result += *coerceToString(noPos, *v2, context,
+                            "while evaluating one element of the list",
+                            coerceMore, copyToStore, canonicalizePath);
+                } catch (Error & e) {
+                    e.addTrace(positions[pos], errorCtx);
+                    throw;
+                }
                 if (n < v.listSize() - 1
                     /* !!! not quite correct */
                     && (!v2->isList() || v2->listSize() != 0))
@@ -2265,14 +2206,16 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         }
     }
 
-    throwTypeError(pos, "cannot coerce %1% to a string", v);
+    error("cannot coerce %1% to a string", showType(v))
+        .withTrace(pos, errorCtx)
+        .debugThrow<TypeError>();
 }
 
 
 StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
 {
     if (nix::isDerivation(path))
-        throwEvalError("file names are not allowed to end in '%1%'", drvExtension);
+        error("file names are not allowed to end in '%1%'", drvExtension).debugThrow<EvalError>();
 
     auto dstPath = [&]() -> StorePath
     {
@@ -2293,28 +2236,25 @@ StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
 }
 
 
-Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context)
+Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
-        throwEvalError(pos, "string '%1%' doesn't represent an absolute path", path);
+        error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
     return path;
 }
 
 
-StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context)
+StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
-    throw EvalError({
-        .msg = hintfmt("path '%1%' is not in the Nix store", path),
-        .errPos = positions[pos]
-    });
+    error("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
 }
 
 
-bool EvalState::eqValues(Value & v1, Value & v2)
+bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_view errorCtx)
 {
     forceValue(v1, noPos);
     forceValue(v2, noPos);
@@ -2334,7 +2274,6 @@ bool EvalState::eqValues(Value & v1, Value & v2)
     if (v1.type() != v2.type()) return false;
 
     switch (v1.type()) {
-
         case nInt:
             return v1.integer == v2.integer;
 
@@ -2353,7 +2292,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
         case nList:
             if (v1.listSize() != v2.listSize()) return false;
             for (size_t n = 0; n < v1.listSize(); ++n)
-                if (!eqValues(*v1.listElems()[n], *v2.listElems()[n])) return false;
+                if (!eqValues(*v1.listElems()[n], *v2.listElems()[n], pos, errorCtx)) return false;
             return true;
 
         case nAttrs: {
@@ -2363,7 +2302,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
                 Bindings::iterator i = v1.attrs->find(sOutPath);
                 Bindings::iterator j = v2.attrs->find(sOutPath);
                 if (i != v1.attrs->end() && j != v2.attrs->end())
-                    return eqValues(*i->value, *j->value);
+                    return eqValues(*i->value, *j->value, pos, errorCtx);
             }
 
             if (v1.attrs->size() != v2.attrs->size()) return false;
@@ -2371,7 +2310,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             /* Otherwise, compare the attributes one by one. */
             Bindings::iterator i, j;
             for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
-                if (i->name != j->name || !eqValues(*i->value, *j->value))
+                if (i->name != j->name || !eqValues(*i->value, *j->value, pos, errorCtx))
                     return false;
 
             return true;
@@ -2388,9 +2327,7 @@ bool EvalState::eqValues(Value & v1, Value & v2)
             return v1.fpoint == v2.fpoint;
 
         default:
-            throwEvalError("cannot compare %1% with %2%",
-                showType(v1),
-                showType(v2));
+            error("cannot compare %1% with %2%", showType(v1), showType(v2)).withTrace(pos, errorCtx).debugThrow<EvalError>();
     }
 }
 
@@ -2517,8 +2454,7 @@ void EvalState::printStats()
 std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType()),
-        .errPos = pos
+        .msg = hintfmt("cannot coerce %1% to a string", showType())
     });
 }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index df6ac431d..e4d5906bd 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -86,6 +86,43 @@ struct DebugTrace {
 
 void debugError(Error * e, Env & env, Expr & expr);
 
+class ErrorBuilder
+{
+    private:
+        EvalState & state;
+        ErrorInfo info;
+
+        ErrorBuilder(EvalState & s, ErrorInfo && i): state(s), info(i) { }
+
+    public:
+        template<typename... Args>
+        [[nodiscard, gnu::noinline]]
+        static ErrorBuilder * create(EvalState & s, const Args & ... args)
+        {
+            return new ErrorBuilder(s, ErrorInfo { .msg = hintfmt(args...) });
+        }
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & atPos(PosIdx pos);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withTrace(PosIdx pos, const std::string_view text);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withFrameTrace(PosIdx pos, const std::string_view text);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withSuggestions(Suggestions & s);
+
+        [[nodiscard, gnu::noinline]]
+        ErrorBuilder & withFrame(const Env & e, const Expr & ex);
+
+        template<class ErrorType>
+        [[gnu::noinline, gnu::noreturn]]
+        void debugThrow();
+};
+
+
 class EvalState : public std::enable_shared_from_this<EvalState>
 {
 public:
@@ -145,29 +182,38 @@ public:
 
     template<class E>
     [[gnu::noinline, gnu::noreturn]]
-    void debugThrow(E && error, const Env & env, const Expr & expr)
+    void debugThrowLastTrace(E && error)
     {
-        if (debugRepl)
-            runDebugRepl(&error, env, expr);
-
-        throw std::move(error);
+        debugThrow(error, nullptr, nullptr);
     }
 
     template<class E>
     [[gnu::noinline, gnu::noreturn]]
-    void debugThrowLastTrace(E && e)
+    void debugThrow(E && error, const Env * env, const Expr * expr)
     {
-        // Call this in the situation where Expr and Env are inaccessible.
-        // The debugger will start in the last context that's in the
-        // DebugTrace stack.
-        if (debugRepl && !debugTraces.empty()) {
-            const DebugTrace & last = debugTraces.front();
-            runDebugRepl(&e, last.env, last.expr);
+        if (debugRepl && ((env && expr) || !debugTraces.empty())) {
+            if (!env || !expr) {
+                const DebugTrace & last = debugTraces.front();
+                env = &last.env;
+                expr = &last.expr;
+            }
+            runDebugRepl(&error, *env, *expr);
         }
 
-        throw std::move(e);
+        throw std::move(error);
     }
 
+    // This is dangerous, but gets in line with the idea that error creation and
+    // throwing should not allocate on the stack of hot functions.
+    // as long as errors are immediately thrown, it works.
+    ErrorBuilder * errorBuilder;
+
+    template<typename... Args>
+    [[nodiscard, gnu::noinline]]
+    ErrorBuilder & error(const Args & ... args) {
+        errorBuilder = ErrorBuilder::create(*this, args...);
+        return *errorBuilder;
+    }
 
 private:
     SrcToStore srcToStore;
@@ -282,8 +328,8 @@ public:
     /* Evaluation the expression, then verify that it has the expected
        type. */
     inline bool evalBool(Env & env, Expr * e);
-    inline bool evalBool(Env & env, Expr * e, const PosIdx pos);
-    inline void evalAttrs(Env & env, Expr * e, Value & v);
+    inline bool evalBool(Env & env, Expr * e, const PosIdx pos, std::string_view errorCtx);
+    inline void evalAttrs(Env & env, Expr * e, Value & v, const PosIdx pos, std::string_view errorCtx);
 
     /* If `v' is a thunk, enter it and overwrite `v' with the result
        of the evaluation of the thunk.  If `v' is a delayed function
@@ -299,89 +345,25 @@ public:
     void forceValueDeep(Value & v);
 
     /* Force `v', and then verify that it has the expected type. */
-    NixInt forceInt(Value & v, const PosIdx pos);
-    NixFloat forceFloat(Value & v, const PosIdx pos);
-    bool forceBool(Value & v, const PosIdx pos);
+    NixInt forceInt(Value & v, const PosIdx pos, std::string_view errorCtx);
+    NixFloat forceFloat(Value & v, const PosIdx pos, std::string_view errorCtx);
+    bool forceBool(Value & v, const PosIdx pos, std::string_view errorCtx);
 
-    void forceAttrs(Value & v, const PosIdx pos);
+    void forceAttrs(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     template <typename Callable>
-    inline void forceAttrs(Value & v, Callable getPos);
-
-    inline void forceList(Value & v, const PosIdx pos);
-    void forceFunction(Value & v, const PosIdx pos); // either lambda or primop
-    std::string_view forceString(Value & v, const PosIdx pos = noPos);
-    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos = noPos);
-    std::string_view forceStringNoCtx(Value & v, const PosIdx pos = noPos);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2, const std::string & s3,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const char * s, const std::string & s2, const std::string & s3);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const char * s, const std::string & s2, const std::string & s3);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx pos, const Suggestions & suggestions, const char * s, const std::string & s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwEvalError(const PosIdx p1, const char * s, const Symbol sym, const PosIdx p2,
-        Env & env, Expr & expr);
+    inline void forceAttrs(Value & v, Callable getPos, std::string_view errorCtx);
 
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const Value & v,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const char * s, const ExprLambda & fun, const Symbol s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const PosIdx pos, const Suggestions & suggestions, const char * s, const ExprLambda & fun, const Symbol s2,
-        Env & env, Expr & expr);
-    [[gnu::noinline, gnu::noreturn]]
-    void throwTypeError(const char * s, const Value & v,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwAssertionError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwUndefinedVarError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
-
-    [[gnu::noinline, gnu::noreturn]]
-    void throwMissingArgumentError(const PosIdx pos, const char * s, const std::string & s1,
-        Env & env, Expr & expr);
+    inline void forceList(Value & v, const PosIdx pos, std::string_view errorCtx);
+    void forceFunction(Value & v, const PosIdx pos, std::string_view errorCtx); // either lambda or primop
+    std::string_view forceString(Value & v, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceString(Value & v, PathSet & context, const PosIdx pos, std::string_view errorCtx);
+    std::string_view forceStringNoCtx(Value & v, const PosIdx pos, std::string_view errorCtx);
 
     [[gnu::noinline]]
     void addErrorTrace(Error & e, const char * s, const std::string & s2) const;
     [[gnu::noinline]]
-    void addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2) const;
+    void addErrorTrace(Error & e, const PosIdx pos, const char * s, const std::string & s2, bool frame = false) const;
 
 public:
     /* Return true iff the value `v' denotes a derivation (i.e. a
@@ -396,6 +378,7 @@ public:
        booleans and lists to a string.  If `copyToStore' is set,
        referenced paths are copied to the Nix store as a side effect. */
     BackedStringView coerceToString(const PosIdx pos, Value & v, PathSet & context,
+        std::string_view errorCtx,
         bool coerceMore = false, bool copyToStore = true,
         bool canonicalizePath = true);
 
@@ -404,10 +387,10 @@ public:
     /* Path coercion.  Converts strings, paths and derivations to a
        path.  The result is guaranteed to be a canonicalised, absolute
        path.  Nothing is copied to the store. */
-    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context);
+    Path coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
 
     /* Like coerceToPath, but the result must be a store path. */
-    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context);
+    StorePath coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx);
 
 public:
 
@@ -467,7 +450,7 @@ public:
 
     /* Do a deep equality test between two values.  That is, list
        elements and attributes are compared recursively. */
-    bool eqValues(Value & v1, Value & v2);
+    bool eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_view errorCtx);
 
     bool isFunctor(Value & fun);
 
@@ -502,7 +485,7 @@ public:
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, PosIdx pos);
 
-    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos);
+    void concatLists(Value & v, size_t nrLists, Value * * lists, const PosIdx pos, std::string_view errorCtx);
 
     /* Print statistics. */
     void printStats();
@@ -665,6 +648,13 @@ extern EvalSettings evalSettings;
 
 static const std::string corepkgsPrefix{"/__corepkgs__/"};
 
+template<class ErrorType>
+void ErrorBuilder::debugThrow()
+{
+    // NOTE: We always use the -LastTrace version as we push the new trace in withFrame()
+    state.debugThrowLastTrace(ErrorType(info));
+}
+
 }
 
 #include "eval-inline.hh"
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 105d32467..336eb274d 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -259,28 +259,28 @@ static Flake getFlake(
             if (setting.value->type() == nString)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    std::string(state.forceStringNoCtx(*setting.value, setting.pos)));
+                    std::string(state.forceStringNoCtx(*setting.value, setting.pos, "")));
             else if (setting.value->type() == nPath) {
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.coerceToString(setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
+                    state.coerceToString(setting.pos, *setting.value, emptyContext, "", false, true, true) .toOwned());
             }
             else if (setting.value->type() == nInt)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    state.forceInt(*setting.value, setting.pos));
+                    state.forceInt(*setting.value, setting.pos, ""));
             else if (setting.value->type() == nBool)
                 flake.config.settings.emplace(
                     state.symbols[setting.name],
-                    Explicit<bool> { state.forceBool(*setting.value, setting.pos) });
+                    Explicit<bool> { state.forceBool(*setting.value, setting.pos, "") });
             else if (setting.value->type() == nList) {
                 std::vector<std::string> ss;
                 for (auto elem : setting.value->listItems()) {
                     if (elem->type() != nString)
                         throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
                             state.symbols[setting.name], showType(*setting.value));
-                    ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos));
+                    ss.emplace_back(state.forceStringNoCtx(*elem, setting.pos, ""));
                 }
                 flake.config.settings.emplace(state.symbols[setting.name], ss);
             }
@@ -741,7 +741,7 @@ void callFlake(EvalState & state,
 
 static void prim_getFlake(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos));
+    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
         throw Error("cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)", flakeRefS, state.positions[pos]);
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 5ad5d1fd4..1602fbffb 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -51,7 +51,7 @@ std::string DrvInfo::queryName() const
     if (name == "" && attrs) {
         auto i = attrs->find(state->sName);
         if (i == attrs->end()) throw TypeError("derivation name missing");
-        name = state->forceStringNoCtx(*i->value);
+        name = state->forceStringNoCtx(*i->value, noPos, "while evaluating the 'name' attribute of a derivation");
     }
     return name;
 }
@@ -61,7 +61,7 @@ std::string DrvInfo::querySystem() const
 {
     if (system == "" && attrs) {
         auto i = attrs->find(state->sSystem);
-        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, i->pos);
+        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, i->pos, "while evaluating the 'system' attribute of a derivation");
     }
     return system;
 }
@@ -75,7 +75,7 @@ std::optional<StorePath> DrvInfo::queryDrvPath() const
         if (i == attrs->end())
             drvPath = {std::nullopt};
         else
-            drvPath = {state->coerceToStorePath(i->pos, *i->value, context)};
+            drvPath = {state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the 'drvPath' attribute of a derivation")};
     }
     return drvPath.value_or(std::nullopt);
 }
@@ -95,7 +95,7 @@ StorePath DrvInfo::queryOutPath() const
         Bindings::iterator i = attrs->find(state->sOutPath);
         PathSet context;
         if (i != attrs->end())
-            outPath = state->coerceToStorePath(i->pos, *i->value, context);
+            outPath = state->coerceToStorePath(i->pos, *i->value, context, "while evaluating the output path of a derivation");
     }
     if (!outPath)
         throw UnimplementedError("CA derivations are not yet supported");
@@ -109,23 +109,23 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         /* Get the ‘outputs’ list. */
         Bindings::iterator i;
         if (attrs && (i = attrs->find(state->sOutputs)) != attrs->end()) {
-            state->forceList(*i->value, i->pos);
+            state->forceList(*i->value, i->pos, "while evaluating the 'outputs' attribute of a derivation");
 
             /* For each output... */
             for (auto elem : i->value->listItems()) {
-                std::string output(state->forceStringNoCtx(*elem, i->pos));
+                std::string output(state->forceStringNoCtx(*elem, i->pos, "while evaluating the name of an output of a derivation"));
 
                 if (withPaths) {
                     /* Evaluate the corresponding set. */
                     Bindings::iterator out = attrs->find(state->symbols.create(output));
                     if (out == attrs->end()) continue; // FIXME: throw error?
-                    state->forceAttrs(*out->value, i->pos);
+                    state->forceAttrs(*out->value, i->pos, "while evaluating an output of a derivation");
 
                     /* And evaluate its ‘outPath’ attribute. */
                     Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
                     if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
                     PathSet context;
-                    outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context));
+                    outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context, "while evaluating an output path of a derivation"));
                 } else
                     outputs.emplace(output, std::nullopt);
             }
@@ -137,7 +137,7 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         return outputs;
 
     Bindings::iterator i;
-    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos, "while evaluating the 'outputSpecified' attribute of a derivation")) {
         Outputs result;
         auto out = outputs.find(queryOutputName());
         if (out == outputs.end())
@@ -169,7 +169,7 @@ std::string DrvInfo::queryOutputName() const
 {
     if (outputName == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutputName);
-        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value) : "";
+        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value, noPos, "while evaluating the output name of a derivation") : "";
     }
     return outputName;
 }
@@ -181,7 +181,7 @@ Bindings * DrvInfo::getMeta()
     if (!attrs) return 0;
     Bindings::iterator a = attrs->find(state->sMeta);
     if (a == attrs->end()) return 0;
-    state->forceAttrs(*a->value, a->pos);
+    state->forceAttrs(*a->value, a->pos, "while evaluating the 'meta' attribute of a derivation");
     meta = a->value->attrs;
     return meta;
 }
@@ -382,7 +382,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    `recurseForDerivations = true' attribute. */
                 if (i->value->type() == nAttrs) {
                     Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
-                    if (j != i->value->attrs->end() && state.forceBool(*j->value, j->pos))
+                    if (j != i->value->attrs->end() && state.forceBool(*j->value, j->pos, "while evaluating the attribute `recurseForDerivations`"))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 }
             }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index ac7ce021e..ffe67f97d 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -8,7 +8,6 @@
 #include "error.hh"
 #include "chunked-vector.hh"
 
-
 namespace nix {
 
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index e07909f8e..ffb364a90 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -400,21 +400,21 @@ expr_op
   | '-' expr_op %prec NEGATE { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {new ExprInt(0), $2}); }
   | expr_op EQ expr_op { $$ = new ExprOpEq($1, $3); }
   | expr_op NEQ expr_op { $$ = new ExprOpNEq($1, $3); }
-  | expr_op '<' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
-  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
-  | expr_op '>' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
-  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
-  | expr_op AND expr_op { $$ = new ExprOpAnd(CUR_POS, $1, $3); }
-  | expr_op OR expr_op { $$ = new ExprOpOr(CUR_POS, $1, $3); }
-  | expr_op IMPL expr_op { $$ = new ExprOpImpl(CUR_POS, $1, $3); }
-  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(CUR_POS, $1, $3); }
+  | expr_op '<' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
+  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
+  | expr_op '>' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
+  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
+  | expr_op AND expr_op { $$ = new ExprOpAnd(makeCurPos(@2, data), $1, $3); }
+  | expr_op OR expr_op { $$ = new ExprOpOr(makeCurPos(@2, data), $1, $3); }
+  | expr_op IMPL expr_op { $$ = new ExprOpImpl(makeCurPos(@2, data), $1, $3); }
+  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(makeCurPos(@2, data), $1, $3); }
   | expr_op '?' attrpath { $$ = new ExprOpHasAttr($1, *$3); }
   | expr_op '+' expr_op
-    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<PosIdx, Expr *>>({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
-  | expr_op '-' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
-  | expr_op '*' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
-  | expr_op '/' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__div")), {$1, $3}); }
-  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(CUR_POS, $1, $3); }
+    { $$ = new ExprConcatStrings(makeCurPos(@2, data), false, new std::vector<std::pair<PosIdx, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
+  | expr_op '-' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
+  | expr_op '*' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
+  | expr_op '/' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__div")), {$1, $3}); }
+  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(makeCurPos(@2, data), $1, $3); }
   | expr_app
   ;
 
@@ -782,13 +782,13 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
     if (hasPrefix(path, "nix/"))
         return concatStrings(corepkgsPrefix, path.substr(4));
 
-    debugThrowLastTrace(ThrownError({
+    debugThrow(ThrownError({
         .msg = hintfmt(evalSettings.pureEval
             ? "cannot look up '<%s>' in pure evaluation mode (use '--impure' to override)"
             : "file '%s' was not found in the Nix search path (add it using $NIX_PATH or -I)",
             path),
         .errPos = positions[pos]
-    }));
+    }), 0, 0);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 080892cbd..9b93e34a2 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -114,15 +114,7 @@ static Path realisePath(EvalState & state, const PosIdx pos, Value & v, const Re
 {
     PathSet context;
 
-    auto path = [&]()
-    {
-        try {
-            return state.coerceToPath(pos, v, context);
-        } catch (Error & e) {
-            e.addTrace(state.positions[pos], "while realising the context of a path");
-            throw;
-        }
-    }();
+    auto path = state.coerceToPath(noPos, v, context, "while realising the context of a path");
 
     try {
         StringMap rewrites = state.realiseContext(context);
@@ -209,9 +201,9 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
                 , "/"), **state.vImportedDrvToDerivation);
         }
 
-        state.forceFunction(**state.vImportedDrvToDerivation, pos);
+        state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
         v.mkApp(*state.vImportedDrvToDerivation, w);
-        state.forceAttrs(v, pos);
+        state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
     }
 
     else if (path == corepkgsPrefix + "fetchurl.nix") {
@@ -224,7 +216,7 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
         if (!vScope)
             state.evalFile(path, v);
         else {
-            state.forceAttrs(*vScope, pos);
+            state.forceAttrs(*vScope, pos, "while evaluating the first argument passed to builtins.scopedImport");
 
             Env * env = &state.allocEnv(vScope->attrs->size());
             env->up = &state.baseEnv;
@@ -329,7 +321,7 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 {
     auto path = realisePath(state, pos, *args[0]);
 
-    std::string sym(state.forceStringNoCtx(*args[1], pos));
+    std::string sym(state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.importNative"));
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
@@ -354,28 +346,26 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
 /* Execute a program and parse its output */
 void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.exec");
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("at least one argument to 'exec' required"),
-            .errPos = state.positions[pos]
-        }));
+        state.error("at least one argument to 'exec' required").atPos(pos).debugThrow<EvalError>();
     PathSet context;
-    auto program = state.coerceToString(pos, *elems[0], context, false, false).toOwned();
+    auto program = state.coerceToString(pos, *elems[0], context,
+            "while evaluating the first element of the argument passed to builtins.exec",
+            false, false).toOwned();
     Strings commandArgs;
     for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
-        commandArgs.push_back(state.coerceToString(pos, *elems[i], context, false, false).toOwned());
+        commandArgs.push_back(
+                state.coerceToString(pos, *elems[i], context,
+                        "while evaluating an element of the argument passed to builtins.exec",
+                        false, false).toOwned());
     }
     try {
         auto _ = state.realiseContext(context); // FIXME: Handle CA derivations
     } catch (InvalidPathError & e) {
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("cannot execute '%1%', since path '%2%' is not valid",
-                program, e.path),
-            .errPos = state.positions[pos]
-        }));
+        state.error("cannot execute '%1%', since path '%2%' is not valid", program, e.path).atPos(pos).debugThrow<EvalError>();
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -383,18 +373,17 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     try {
         parsed = state.parseExprFromString(std::move(output), "/");
     } catch (Error & e) {
-        e.addTrace(state.positions[pos], "While parsing the output from '%1%'", program);
+        e.addTrace(state.positions[pos], "while parsing the output from '%1%'", program);
         throw;
     }
     try {
         state.eval(parsed, v);
     } catch (Error & e) {
-        e.addTrace(state.positions[pos], "While evaluating the output from '%1%'", program);
+        e.addTrace(state.positions[pos], "while evaluating the output from '%1%'", program);
         throw;
     }
 }
 
-
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
@@ -545,42 +534,69 @@ static RegisterPrimOp primop_isPath({
     .fun = prim_isPath,
 });
 
+template<typename Callable>
+ static inline void withExceptionContext(Trace trace, Callable&& func)
+{
+    try
+    {
+        func();
+    }
+    catch(Error & e)
+    {
+        e.pushTrace(trace);
+        throw;
+    }
+}
+
 struct CompareValues
 {
     EvalState & state;
+    const PosIdx pos;
+    const std::string_view errorCtx;
 
-    CompareValues(EvalState & state) : state(state) { };
+    CompareValues(EvalState & state, const PosIdx pos, const std::string_view && errorCtx) : state(state), pos(pos), errorCtx(errorCtx) { };
 
     bool operator () (Value * v1, Value * v2) const
     {
-        if (v1->type() == nFloat && v2->type() == nInt)
-            return v1->fpoint < v2->integer;
-        if (v1->type() == nInt && v2->type() == nFloat)
-            return v1->integer < v2->fpoint;
-        if (v1->type() != v2->type())
-            state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
-        switch (v1->type()) {
-            case nInt:
-                return v1->integer < v2->integer;
-            case nFloat:
-                return v1->fpoint < v2->fpoint;
-            case nString:
-                return strcmp(v1->string.s, v2->string.s) < 0;
-            case nPath:
-                return strcmp(v1->path, v2->path) < 0;
-            case nList:
-                // Lexicographic comparison
-                for (size_t i = 0;; i++) {
-                    if (i == v2->listSize()) {
-                        return false;
-                    } else if (i == v1->listSize()) {
-                        return true;
-                    } else if (!state.eqValues(*v1->listElems()[i], *v2->listElems()[i])) {
-                        return (*this)(v1->listElems()[i], v2->listElems()[i]);
+        return (*this)(v1, v2, errorCtx);
+    }
+
+    bool operator () (Value * v1, Value * v2, std::string_view errorCtx) const
+    {
+        try {
+            if (v1->type() == nFloat && v2->type() == nInt)
+                return v1->fpoint < v2->integer;
+            if (v1->type() == nInt && v2->type() == nFloat)
+                return v1->integer < v2->fpoint;
+            if (v1->type() != v2->type())
+                state.error("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow<EvalError>();
+            switch (v1->type()) {
+                case nInt:
+                    return v1->integer < v2->integer;
+                case nFloat:
+                    return v1->fpoint < v2->fpoint;
+                case nString:
+                    return strcmp(v1->string.s, v2->string.s) < 0;
+                case nPath:
+                    return strcmp(v1->path, v2->path) < 0;
+                case nList:
+                    // Lexicographic comparison
+                    for (size_t i = 0;; i++) {
+                        if (i == v2->listSize()) {
+                            return false;
+                        } else if (i == v1->listSize()) {
+                            return true;
+                        } else if (!state.eqValues(*v1->listElems()[i], *v2->listElems()[i], pos, errorCtx)) {
+                            return (*this)(v1->listElems()[i], v2->listElems()[i], "while comparing two list elements");
+                        }
                     }
-                }
-            default:
-                state.debugThrowLastTrace(EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2)));
+                default:
+                    state.error("cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2)).debugThrow<EvalError>();
+            }
+        } catch (Error & e) {
+            if (!errorCtx.empty())
+                e.addTrace(nullptr, errorCtx);
+            throw;
         }
     }
 };
@@ -595,105 +611,67 @@ typedef std::list<Value *> ValueList;
 
 static Bindings::iterator getAttr(
     EvalState & state,
-    std::string_view funcName,
     Symbol attrSym,
     Bindings * attrSet,
-    const PosIdx pos)
+    std::string_view errorCtx)
 {
     Bindings::iterator value = attrSet->find(attrSym);
     if (value == attrSet->end()) {
-        hintformat errorMsg = hintfmt(
-            "attribute '%s' missing for call to '%s'",
-            state.symbols[attrSym],
-            funcName
-        );
-
-        auto aPos = attrSet->pos;
-        if (!aPos) {
-            state.debugThrowLastTrace(TypeError({
-                .msg = errorMsg,
-                .errPos = state.positions[pos],
-            }));
-        } else {
-            auto e = TypeError({
-                .msg = errorMsg,
-                .errPos = state.positions[aPos],
-            });
-
-            // Adding another trace for the function name to make it clear
-            // which call received wrong arguments.
-            e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", funcName));
-            state.debugThrowLastTrace(e);
-        }
+        state.error("attribute '%s' missing", state.symbols[attrSym]).withTrace(noPos, errorCtx).debugThrow<TypeError>();
     }
-
     return value;
 }
 
 static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], noPos, "while evaluating the first argument passed to builtins.genericClosure");
 
     /* Get the start set. */
-    Bindings::iterator startSet = getAttr(
-        state,
-        "genericClosure",
-        state.sStartSet,
-        args[0]->attrs,
-        pos
-    );
+    Bindings::iterator startSet = getAttr(state, state.sStartSet, args[0]->attrs, "in the attrset passed as argument to builtins.genericClosure");
 
-    state.forceList(*startSet->value, pos);
+    state.forceList(*startSet->value, noPos, "while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure");
 
     ValueList workSet;
     for (auto elem : startSet->value->listItems())
         workSet.push_back(elem);
 
-    /* Get the operator. */
-    Bindings::iterator op = getAttr(
-        state,
-        "genericClosure",
-        state.sOperator,
-        args[0]->attrs,
-        pos
-    );
+    if (startSet->value->listSize() == 0) {
+        v = *startSet->value;
+        return;
+    }
 
-    state.forceValue(*op->value, pos);
+    /* Get the operator. */
+    Bindings::iterator op = getAttr(state, state.sOperator, args[0]->attrs, "in the attrset passed as argument to builtins.genericClosure");
+    state.forceFunction(*op->value, noPos, "while evaluating the 'operator' attribute passed as argument to builtins.genericClosure");
 
-    /* Construct the closure by applying the operator to element of
+    /* Construct the closure by applying the operator to elements of
        `workSet', adding the result to `workSet', continuing until
        no new elements are found. */
     ValueList res;
     // `doneKeys' doesn't need to be a GC root, because its values are
     // reachable from res.
-    auto cmp = CompareValues(state);
+    auto cmp = CompareValues(state, noPos, "while comparing the `key` attributes of two genericClosure elements");
     std::set<Value *, decltype(cmp)> doneKeys(cmp);
     while (!workSet.empty()) {
         Value * e = *(workSet.begin());
         workSet.pop_front();
 
-        state.forceAttrs(*e, pos);
+        state.forceAttrs(*e, noPos, "while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure");
 
-        Bindings::iterator key =
-            e->attrs->find(state.sKey);
-        if (key == e->attrs->end())
-            state.debugThrowLastTrace(EvalError({
-                .msg = hintfmt("attribute 'key' required"),
-                .errPos = state.positions[pos]
-            }));
-        state.forceValue(*key->value, pos);
+        Bindings::iterator key = getAttr(state, state.sKey, e->attrs, "in one of the attrsets generated by (or initially passed to) builtins.genericClosure");
+        state.forceValue(*key->value, noPos);
 
         if (!doneKeys.insert(key->value).second) continue;
         res.push_back(e);
 
         /* Call the `operator' function with `e' as argument. */
-        Value call;
-        call.mkApp(op->value, e);
-        state.forceList(call, pos);
+        Value newElements;
+        state.callFunction(*op->value, 1, &e, newElements, noPos);
+        state.forceList(newElements, noPos, "while evaluating the return value of the `operator` passed to builtins.genericClosure");
 
         /* Add the values returned by the operator to the work set. */
-        for (auto elem : call.listItems()) {
-            state.forceValue(*elem, pos);
+        for (auto elem : newElements.listItems()) {
+            state.forceValue(*elem, noPos); // "while evaluating one one of the elements returned by the `operator` passed to builtins.genericClosure");
             workSet.push_back(elem);
         }
     }
@@ -761,7 +739,7 @@ static RegisterPrimOp primop_break({
                 throw Error(ErrorInfo{
                     .level = lvlInfo,
                     .msg = hintfmt("quit the debugger"),
-                    .errPos = state.positions[noPos],
+                    .errPos = nullptr,
                 });
             }
         }
@@ -780,7 +758,8 @@ static RegisterPrimOp primop_abort({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
         PathSet context;
-        auto s = state.coerceToString(pos, *args[0], context).toOwned();
+        auto s = state.coerceToString(pos, *args[0], context,
+                "while evaluating the error message passed to builtins.abort").toOwned();
         state.debugThrowLastTrace(Abort("evaluation aborted with the following error message: '%1%'", s));
     }
 });
@@ -798,7 +777,8 @@ static RegisterPrimOp primop_throw({
     .fun = [](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
       PathSet context;
-      auto s = state.coerceToString(pos, *args[0], context).toOwned();
+      auto s = state.coerceToString(pos, *args[0], context,
+              "while evaluating the error message passed to builtin.throw").toOwned();
       state.debugThrowLastTrace(ThrownError(s));
     }
 });
@@ -810,7 +790,10 @@ static void prim_addErrorContext(EvalState & state, const PosIdx pos, Value * *
         v = *args[1];
     } catch (Error & e) {
         PathSet context;
-        e.addTrace(nullptr, state.coerceToString(pos, *args[0], context).toOwned());
+        auto message = state.coerceToString(pos, *args[0], context,
+                "while evaluating the error message passed to builtins.addErrorContext",
+                false, false).toOwned();
+        e.addTrace(nullptr, message, true);
         throw;
     }
 }
@@ -823,7 +806,8 @@ static RegisterPrimOp primop_addErrorContext(RegisterPrimOp::Info {
 
 static void prim_ceil(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos),
+            "while evaluating the first argument passed to builtins.ceil");
     v.mkInt(ceil(value));
 }
 
@@ -842,7 +826,7 @@ static RegisterPrimOp primop_ceil({
 
 static void prim_floor(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos), "while evaluating the first argument passed to builtins.floor");
     v.mkInt(floor(value));
 }
 
@@ -916,7 +900,7 @@ static RegisterPrimOp primop_tryEval({
 /* Return an environment variable.  Use with care. */
 static void prim_getEnv(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    std::string name(state.forceStringNoCtx(*args[0], pos));
+    std::string name(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.getEnv"));
     v.mkString(evalSettings.restrictEval || evalSettings.pureEval ? "" : getEnv(name).value_or(""));
 }
 
@@ -1013,6 +997,7 @@ static void prim_second(EvalState & state, const PosIdx pos, Value * * args, Val
  * Derivations
  *************************************************************/
 
+static void derivationStrictInternal(EvalState & state, const std::string & name, Bindings * attrs, Value & v);
 
 /* Construct (as a unobservable side effect) a Nix derivation
    expression that performs the derivation described by the argument
@@ -1023,38 +1008,68 @@ static void prim_second(EvalState & state, const PosIdx pos, Value * * args, Val
    derivation. */
 static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    using nlohmann::json;
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.derivationStrict");
+
+    Bindings * attrs = args[0]->attrs;
 
     /* Figure out the name first (for stack backtraces). */
-    Bindings::iterator attr = getAttr(
-        state,
-        "derivationStrict",
-        state.sName,
-        args[0]->attrs,
-        pos
-    );
+    Bindings::iterator nameAttr = getAttr(state, state.sName, attrs, "in the attrset passed as argument to builtins.derivationStrict");
 
     std::string drvName;
-    const auto posDrvName = attr->pos;
     try {
-        drvName = state.forceStringNoCtx(*attr->value, pos);
+        drvName = state.forceStringNoCtx(*nameAttr->value, pos, "while evaluating the `name` attribute passed to builtins.derivationStrict");
+    } catch (Error & e) {
+        e.addTrace(state.positions[nameAttr->pos], "while evaluating the derivation attribute 'name'");
+        throw;
+    }
+
+    try {
+        derivationStrictInternal(state, drvName, attrs, v);
     } catch (Error & e) {
-        e.addTrace(state.positions[posDrvName], "while evaluating the derivation attribute 'name'");
+        Pos pos = state.positions[nameAttr->pos];
+        /*
+         * Here we make two abuses of the error system
+         *
+         * 1. We print the location as a string to avoid a code snippet being
+         * printed. While the location of the name attribute is a good hint, the
+         * exact code there is irrelevant.
+         *
+         * 2. We mark this trace as a frame trace, meaning that we stop printing
+         * less important traces from now on. In particular, this prevents the
+         * display of the automatic "while calling builtins.derivationStrict"
+         * trace, which is of little use for the public we target here.
+         *
+         * Please keep in mind that error reporting is done on a best-effort
+         * basis in nix. There is no accurate location for a derivation, as it
+         * often results from the composition of several functions
+         * (derivationStrict, derivation, mkDerivation, mkPythonModule, etc.)
+         */
+        e.addTrace(nullptr, hintfmt(
+                "while evaluating derivation '%s'\n"
+                "  whose name attribute is located at %s",
+                drvName, pos), true);
         throw;
     }
+}
 
+static void derivationStrictInternal(EvalState & state, const std::string &
+drvName, Bindings * attrs, Value & v)
+{
     /* Check whether attributes should be passed as a JSON file. */
+    using nlohmann::json;
     std::optional<json> jsonObject;
-    attr = args[0]->attrs->find(state.sStructuredAttrs);
-    if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos))
+    auto attr = attrs->find(state.sStructuredAttrs);
+    if (attr != attrs->end() &&
+        state.forceBool(*attr->value, noPos,
+                        "while evaluating the `__structuredAttrs` "
+                        "attribute passed to builtins.derivationStrict"))
         jsonObject = json::object();
 
     /* Check whether null attributes should be ignored. */
     bool ignoreNulls = false;
-    attr = args[0]->attrs->find(state.sIgnoreNulls);
-    if (attr != args[0]->attrs->end())
-        ignoreNulls = state.forceBool(*attr->value, pos);
+    attr = attrs->find(state.sIgnoreNulls);
+    if (attr != attrs->end())
+        ignoreNulls = state.forceBool(*attr->value, noPos, "while evaluating the `__ignoreNulls` attribute " "passed to builtins.derivationStrict");
 
     /* Build the derivation expression by processing the attributes. */
     Derivation drv;
@@ -1071,7 +1086,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     StringSet outputs;
     outputs.insert("out");
 
-    for (auto & i : args[0]->attrs->lexicographicOrder(state.symbols)) {
+    for (auto & i : attrs->lexicographicOrder(state.symbols)) {
         if (i->name == state.sIgnoreNulls) continue;
         const std::string & key = state.symbols[i->name];
         vomit("processing attribute '%1%'", key);
@@ -1082,7 +1097,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             else
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
-                    .errPos = state.positions[posDrvName]
+                    .errPos = state.positions[noPos]
                 }));
         };
 
@@ -1092,7 +1107,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (outputs.find(j) != outputs.end())
                     state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("duplicate derivation output '%1%'", j),
-                        .errPos = state.positions[posDrvName]
+                        .errPos = state.positions[noPos]
                     }));
                 /* !!! Check whether j is a valid attribute
                    name. */
@@ -1102,32 +1117,35 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (j == "drv")
                     state.debugThrowLastTrace(EvalError({
                         .msg = hintfmt("invalid derivation output name 'drv'" ),
-                        .errPos = state.positions[posDrvName]
+                        .errPos = state.positions[noPos]
                     }));
                 outputs.insert(j);
             }
             if (outputs.empty())
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("derivation cannot have an empty set of outputs"),
-                    .errPos = state.positions[posDrvName]
+                    .errPos = state.positions[noPos]
                 }));
         };
 
         try {
+            // This try-catch block adds context for most errors.
+            // Use this empty error context to signify that we defer to it.
+            const std::string_view context_below("");
 
             if (ignoreNulls) {
-                state.forceValue(*i->value, pos);
+                state.forceValue(*i->value, noPos);
                 if (i->value->type() == nNull) continue;
             }
 
             if (i->name == state.sContentAddressed) {
-                contentAddressed = state.forceBool(*i->value, pos);
+                contentAddressed = state.forceBool(*i->value, noPos, context_below);
                 if (contentAddressed)
                     settings.requireExperimentalFeature(Xp::CaDerivations);
             }
 
             else if (i->name == state.sImpure) {
-                isImpure = state.forceBool(*i->value, pos);
+                isImpure = state.forceBool(*i->value, noPos, context_below);
                 if (isImpure)
                     settings.requireExperimentalFeature(Xp::ImpureDerivations);
             }
@@ -1135,9 +1153,11 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
             else if (i->name == state.sArgs) {
-                state.forceList(*i->value, pos);
+                state.forceList(*i->value, noPos, context_below);
                 for (auto elem : i->value->listItems()) {
-                    auto s = state.coerceToString(posDrvName, *elem, context, true).toOwned();
+                    auto s = state.coerceToString(noPos, *elem, context,
+                            "while evaluating an element of the argument list",
+                            true).toOwned();
                     drv.args.push_back(s);
                 }
             }
@@ -1150,29 +1170,29 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
 
                     if (i->name == state.sStructuredAttrs) continue;
 
-                    (*jsonObject)[key] = printValueAsJSON(state, true, *i->value, pos, context);
+                    (*jsonObject)[key] = printValueAsJSON(state, true, *i->value, noPos, context);
 
                     if (i->name == state.sBuilder)
-                        drv.builder = state.forceString(*i->value, context, posDrvName);
+                        drv.builder = state.forceString(*i->value, context, noPos, context_below);
                     else if (i->name == state.sSystem)
-                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName);
+                        drv.platform = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHash)
-                        outputHash = state.forceStringNoCtx(*i->value, posDrvName);
+                        outputHash = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashAlgo)
-                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName);
+                        outputHashAlgo = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashMode)
-                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName));
+                        handleHashMode(state.forceStringNoCtx(*i->value, noPos, context_below));
                     else if (i->name == state.sOutputs) {
                         /* Require ‘outputs’ to be a list of strings. */
-                        state.forceList(*i->value, posDrvName);
+                        state.forceList(*i->value, noPos, context_below);
                         Strings ss;
                         for (auto elem : i->value->listItems())
-                            ss.emplace_back(state.forceStringNoCtx(*elem, posDrvName));
+                            ss.emplace_back(state.forceStringNoCtx(*elem, noPos, context_below));
                         handleOutputs(ss);
                     }
 
                 } else {
-                    auto s = state.coerceToString(i->pos, *i->value, context, true).toOwned();
+                    auto s = state.coerceToString(noPos, *i->value, context, context_below, true).toOwned();
                     drv.env.emplace(key, s);
                     if (i->name == state.sBuilder) drv.builder = std::move(s);
                     else if (i->name == state.sSystem) drv.platform = std::move(s);
@@ -1186,9 +1206,9 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             }
 
         } catch (Error & e) {
-            e.addTrace(state.positions[posDrvName],
-                "while evaluating the attribute '%1%' of the derivation '%2%'",
-                key, drvName);
+            e.addTrace(state.positions[i->pos],
+                hintfmt("while evaluating attribute '%1%' of derivation '%2%'", key, drvName),
+                true);
             throw;
         }
     }
@@ -1232,20 +1252,20 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
     if (drv.builder == "")
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'builder' missing"),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     if (drv.platform == "")
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("required attribute 'system' missing"),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
-            .errPos = state.positions[posDrvName]
+            .errPos = state.positions[noPos]
         }));
 
     if (outputHash) {
@@ -1256,7 +1276,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
             state.debugThrowLastTrace(Error({
                 .msg = hintfmt("multiple outputs are not supported in fixed-output derivations"),
-                .errPos = state.positions[posDrvName]
+                .errPos = state.positions[noPos]
             }));
 
         auto h = newHashAllowEmpty(*outputHash, parseHashTypeOpt(outputHashAlgo));
@@ -1277,7 +1297,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         if (contentAddressed && isImpure)
             throw EvalError({
                 .msg = hintfmt("derivation cannot be both content-addressed and impure"),
-                .errPos = state.positions[posDrvName]
+                .errPos = state.positions[noPos]
             });
 
         auto ht = parseHashTypeOpt(outputHashAlgo).value_or(htSHA256);
@@ -1321,7 +1341,7 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
                 if (!h)
                     throw AssertionError({
                         .msg = hintfmt("derivation produced no hash for output '%s'", i),
-                        .errPos = state.positions[posDrvName],
+                        .errPos = state.positions[noPos],
                     });
                 auto outPath = state.store->makeOutputPath(i, *h, drvName);
                 drv.env[i] = state.store->printStorePath(outPath);
@@ -1354,11 +1374,12 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
         drvHashes.lock()->insert_or_assign(drvPath, h);
     }
 
-    auto attrs = state.buildBindings(1 + drv.outputs.size());
-    attrs.alloc(state.sDrvPath).mkString(drvPathS, {"=" + drvPathS});
+    auto result = state.buildBindings(1 + drv.outputs.size());
+    result.alloc(state.sDrvPath).mkString(drvPathS, {"=" + drvPathS});
     for (auto & i : drv.outputs)
-        mkOutputString(state, attrs, drvPath, drv, i);
-    v.mkAttrs(attrs);
+        mkOutputString(state, result, drvPath, drv, i);
+
+    v.mkAttrs(result);
 }
 
 static RegisterPrimOp primop_derivationStrict(RegisterPrimOp::Info {
@@ -1376,7 +1397,7 @@ static RegisterPrimOp primop_derivationStrict(RegisterPrimOp::Info {
    ‘out’. */
 static void prim_placeholder(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkString(hashPlaceholder(state.forceStringNoCtx(*args[0], pos)));
+    v.mkString(hashPlaceholder(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.placeholder")));
 }
 
 static RegisterPrimOp primop_placeholder({
@@ -1400,7 +1421,7 @@ static RegisterPrimOp primop_placeholder({
 static void prim_toPath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[0], context);
+    Path path = state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.toPath");
     v.mkString(canonPath(path), context);
 }
 
@@ -1431,7 +1452,7 @@ static void prim_storePath(EvalState & state, const PosIdx pos, Value * * args,
         }));
 
     PathSet context;
-    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
+    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context, "while evaluating the first argument passed to builtins.storePath"));
     /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
        directly in the store.  The latter condition is necessary so
        e.g. nix-push does the right thing. */
@@ -1501,7 +1522,9 @@ static RegisterPrimOp primop_pathExists({
 static void prim_baseNameOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context, false, false)), context);
+    v.mkString(baseNameOf(*state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.baseNameOf",
+            false, false)), context);
 }
 
 static RegisterPrimOp primop_baseNameOf({
@@ -1521,7 +1544,9 @@ static RegisterPrimOp primop_baseNameOf({
 static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto path = state.coerceToString(pos, *args[0], context, false, false);
+    auto path = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.dirOf",
+            false, false);
     auto dir = dirOf(*path);
     if (args[0]->type() == nPath) v.mkPath(dir); else v.mkString(dir, context);
 }
@@ -1572,28 +1597,24 @@ static RegisterPrimOp primop_readFile({
    which are desugared to 'findFile __nixPath "x"'. */
 static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.findFile");
 
     SearchPath searchPath;
 
     for (auto v2 : args[0]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "while evaluating an element of the list passed to builtins.findFile");
 
         std::string prefix;
         Bindings::iterator i = v2->attrs->find(state.sPrefix);
         if (i != v2->attrs->end())
-            prefix = state.forceStringNoCtx(*i->value, pos);
+            prefix = state.forceStringNoCtx(*i->value, pos, "while evaluating the `prefix` attribute of an element of the list passed to builtins.findFile");
 
-        i = getAttr(
-            state,
-            "findFile",
-            state.sPath,
-            v2->attrs,
-            pos
-        );
+        i = getAttr(state, state.sPath, v2->attrs, "in an element of the __nixPath");
 
         PathSet context;
-        auto path = state.coerceToString(pos, *i->value, context, false, false).toOwned();
+        auto path = state.coerceToString(pos, *i->value, context,
+                "while evaluating the `path` attribute of an element of the list passed to builtins.findFile",
+                false, false).toOwned();
 
         try {
             auto rewrites = state.realiseContext(context);
@@ -1608,7 +1629,7 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
         searchPath.emplace_back(prefix, path);
     }
 
-    auto path = state.forceStringNoCtx(*args[1], pos);
+    auto path = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.findFile");
 
     v.mkPath(state.checkSourcePath(state.findFile(searchPath, path, pos)));
 }
@@ -1622,7 +1643,7 @@ static RegisterPrimOp primop_findFile(RegisterPrimOp::Info {
 /* Return the cryptographic hash of a file in base-16. */
 static void prim_hashFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto type = state.forceStringNoCtx(*args[0], pos);
+    auto type = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.hashFile");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
         state.debugThrowLastTrace(Error({
@@ -1829,7 +1850,7 @@ static RegisterPrimOp primop_toJSON({
 /* Parse a JSON string to a value. */
 static void prim_fromJSON(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto s = state.forceStringNoCtx(*args[0], pos);
+    auto s = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.fromJSON");
     try {
         parseJSON(state, s, v);
     } catch (JSONParseError &e) {
@@ -1858,8 +1879,8 @@ static RegisterPrimOp primop_fromJSON({
 static void prim_toFile(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    std::string name(state.forceStringNoCtx(*args[0], pos));
-    std::string contents(state.forceString(*args[1], context, pos));
+    std::string name(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.toFile"));
+    std::string contents(state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.toFile"));
 
     StorePathSet refs;
 
@@ -2016,7 +2037,7 @@ static void addPath(
             Value res;
             state.callFunction(*filterFun, 2, args, res, pos);
 
-            return state.forceBool(res, pos);
+            return state.forceBool(res, pos, "while evaluating the return value of the path filter function");
         }) : defaultPathFilter;
 
         std::optional<StorePath> expectedStorePath;
@@ -2042,17 +2063,8 @@ static void addPath(
 static void prim_filterSource(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    Path path = state.coerceToPath(pos, *args[1], context);
-
-    state.forceValue(*args[0], pos);
-    if (args[0]->type() != nFunction)
-        state.debugThrowLastTrace(TypeError({
-            .msg = hintfmt(
-                "first argument in call to 'filterSource' is not a function but %1%",
-                showType(*args[0])),
-            .errPos = state.positions[pos]
-        }));
-
+    Path path = state.coerceToPath(pos, *args[1], context, "while evaluating the second argument (the path to filter) passed to builtins.filterSource");
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filterSource");
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], FileIngestionMethod::Recursive, std::nullopt, v, context);
 }
 
@@ -2113,7 +2125,7 @@ static RegisterPrimOp primop_filterSource({
 
 static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.path");
     Path path;
     std::string name;
     Value * filterFun = nullptr;
@@ -2124,16 +2136,15 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
     for (auto & attr : *args[0]->attrs) {
         auto n = state.symbols[attr.name];
         if (n == "path")
-            path = state.coerceToPath(attr.pos, *attr.value, context);
+            path = state.coerceToPath(attr.pos, *attr.value, context, "while evaluating the `path` attribute passed to builtins.path");
         else if (attr.name == state.sName)
-            name = state.forceStringNoCtx(*attr.value, attr.pos);
-        else if (n == "filter") {
-            state.forceValue(*attr.value, pos);
-            filterFun = attr.value;
-        } else if (n == "recursive")
-            method = FileIngestionMethod { state.forceBool(*attr.value, attr.pos) };
+            name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.path");
+        else if (n == "filter")
+            state.forceFunction(*(filterFun = attr.value), attr.pos, "while evaluating the `filter` parameter passed to builtins.path");
+        else if (n == "recursive")
+            method = FileIngestionMethod { state.forceBool(*attr.value, attr.pos, "while evaluating the `recursive` attribute passed to builtins.path") };
         else if (n == "sha256")
-            expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos), htSHA256);
+            expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `sha256` attribute passed to builtins.path"), htSHA256);
         else
             state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("unsupported argument '%1%' to 'addPath'", state.symbols[attr.name]),
@@ -2142,7 +2153,7 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
     }
     if (path.empty())
         state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("'path' required"),
+            .msg = hintfmt("missing required 'path' attribute in the first argument to builtins.path"),
             .errPos = state.positions[pos]
         }));
     if (name.empty())
@@ -2196,7 +2207,7 @@ static RegisterPrimOp primop_path({
    strings. */
 static void prim_attrNames(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrNames");
 
     state.mkList(v, args[0]->attrs->size());
 
@@ -2223,7 +2234,7 @@ static RegisterPrimOp primop_attrNames({
    order as attrNames. */
 static void prim_attrValues(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.attrValues");
 
     state.mkList(v, args[0]->attrs->size());
 
@@ -2255,14 +2266,13 @@ static RegisterPrimOp primop_attrValues({
 /* Dynamic version of the `.' operator. */
 void prim_getAttr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.getAttr");
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.getAttr");
     Bindings::iterator i = getAttr(
         state,
-        "getAttr",
         state.symbols.create(attr),
         args[1]->attrs,
-        pos
+        "in the attribute set under consideration"
     );
     // !!! add to stack trace?
     if (state.countCalls && i->pos) state.attrSelects[i->pos]++;
@@ -2285,8 +2295,8 @@ static RegisterPrimOp primop_getAttr({
 /* Return position information of the specified attribute. */
 static void prim_unsafeGetAttrPos(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.unsafeGetAttrPos");
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.unsafeGetAttrPos");
     Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
     if (i == args[1]->attrs->end())
         v.mkNull();
@@ -2303,8 +2313,8 @@ static RegisterPrimOp primop_unsafeGetAttrPos(RegisterPrimOp::Info {
 /* Dynamic version of the `?' operator. */
 static void prim_hasAttr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.hasAttr");
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.hasAttr");
     v.mkBool(args[1]->attrs->find(state.symbols.create(attr)) != args[1]->attrs->end());
 }
 
@@ -2337,8 +2347,8 @@ static RegisterPrimOp primop_isAttrs({
 
 static void prim_removeAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the first argument passed to builtins.removeAttrs");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.removeAttrs");
 
     /* Get the attribute names to be removed.
        We keep them as Attrs instead of Symbols so std::set_difference
@@ -2346,7 +2356,7 @@ static void prim_removeAttrs(EvalState & state, const PosIdx pos, Value * * args
     boost::container::small_vector<Attr, 64> names;
     names.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
-        state.forceStringNoCtx(*elem, pos);
+        state.forceStringNoCtx(*elem, pos, "while evaluating the values of the second argument passed to builtins.removeAttrs");
         names.emplace_back(state.symbols.create(elem->string.s), nullptr);
     }
     std::sort(names.begin(), names.end());
@@ -2385,34 +2395,22 @@ static RegisterPrimOp primop_removeAttrs({
    name, the first takes precedence. */
 static void prim_listToAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "while evaluating the argument passed to builtins.listToAttrs");
 
     auto attrs = state.buildBindings(args[0]->listSize());
 
     std::set<Symbol> seen;
 
     for (auto v2 : args[0]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "while evaluating an element of the list passed to builtins.listToAttrs");
 
-        Bindings::iterator j = getAttr(
-            state,
-            "listToAttrs",
-            state.sName,
-            v2->attrs,
-            pos
-        );
+        Bindings::iterator j = getAttr(state, state.sName, v2->attrs, "in a {name=...; value=...;} pair");
 
-        auto name = state.forceStringNoCtx(*j->value, j->pos);
+        auto name = state.forceStringNoCtx(*j->value, j->pos, "while evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs");
 
         auto sym = state.symbols.create(name);
         if (seen.insert(sym).second) {
-            Bindings::iterator j2 = getAttr(
-                state,
-                "listToAttrs",
-                state.sValue,
-                v2->attrs,
-                pos
-            );
+            Bindings::iterator j2 = getAttr(state, state.sValue, v2->attrs, "in a {name=...; value=...;} pair");
             attrs.insert(sym, j2->value, j2->pos);
         }
     }
@@ -2453,8 +2451,8 @@ static RegisterPrimOp primop_listToAttrs({
 
 static void prim_intersectAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the first argument passed to builtins.intersectAttrs");
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.intersectAttrs");
 
     Bindings &left = *args[0]->attrs;
     Bindings &right = *args[1]->attrs;
@@ -2531,14 +2529,14 @@ static RegisterPrimOp primop_intersectAttrs({
 
 static void prim_catAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos));
-    state.forceList(*args[1], pos);
+    auto attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.catAttrs"));
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.catAttrs");
 
     Value * res[args[1]->listSize()];
     unsigned int found = 0;
 
     for (auto v2 : args[1]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "while evaluating an element in the list passed as second argument to builtins.catAttrs");
         Bindings::iterator i = v2->attrs->find(attrName);
         if (i != v2->attrs->end())
             res[found++] = i->value;
@@ -2611,7 +2609,7 @@ static RegisterPrimOp primop_functionArgs({
 /*  */
 static void prim_mapAttrs(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.mapAttrs");
 
     auto attrs = state.buildBindings(args[1]->attrs->size());
 
@@ -2652,21 +2650,16 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
 
     std::map<Symbol, std::pair<size_t, Value * *>> attrsSeen;
 
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.zipAttrsWith");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.zipAttrsWith");
     const auto listSize = args[1]->listSize();
     const auto listElems = args[1]->listElems();
 
     for (unsigned int n = 0; n < listSize; ++n) {
         Value * vElem = listElems[n];
-        try {
-            state.forceAttrs(*vElem, noPos);
-            for (auto & attr : *vElem->attrs)
-                attrsSeen[attr.name].first++;
-        } catch (TypeError & e) {
-            e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "zipAttrsWith"));
-            state.debugThrowLastTrace(e);
-        }
+        state.forceAttrs(*vElem, noPos, "while evaluating a value of the list passed as second argument to builtins.zipAttrsWith");
+        for (auto & attr : *vElem->attrs)
+            attrsSeen[attr.name].first++;
     }
 
     auto attrs = state.buildBindings(attrsSeen.size());
@@ -2750,7 +2743,7 @@ static RegisterPrimOp primop_isList({
 
 static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Value & v)
 {
-    state.forceList(list, pos);
+    state.forceList(list, pos, "while evaluating the first argument passed to builtins.elemAt");
     if (n < 0 || (unsigned int) n >= list.listSize())
         state.debugThrowLastTrace(Error({
             .msg = hintfmt("list index %1% is out of bounds", n),
@@ -2763,7 +2756,7 @@ static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Val
 /* Return the n-1'th element of a list. */
 static void prim_elemAt(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos), v);
+    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.elemAt"), v);
 }
 
 static RegisterPrimOp primop_elemAt({
@@ -2798,7 +2791,7 @@ static RegisterPrimOp primop_head({
    don't want to use it!  */
 static void prim_tail(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.tail");
     if (args[0]->listSize() == 0)
         state.debugThrowLastTrace(Error({
             .msg = hintfmt("'tail' called on an empty list"),
@@ -2829,10 +2822,16 @@ static RegisterPrimOp primop_tail({
 /* Apply a function to every element of a list. */
 static void prim_map(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.map");
 
-    state.mkList(v, args[1]->listSize());
+    if (args[1]->listSize() == 0) {
+        v = *args[1];
+        return;
+    }
+
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.map");
 
+    state.mkList(v, args[1]->listSize());
     for (unsigned int n = 0; n < v.listSize(); ++n)
         (v.listElems()[n] = state.allocValue())->mkApp(
             args[0], args[1]->listElems()[n]);
@@ -2859,8 +2858,14 @@ static RegisterPrimOp primop_map({
    returns true. */
 static void prim_filter(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.filter");
+
+    if (args[1]->listSize() == 0) {
+        v = *args[1];
+        return;
+    }
+
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.filter");
 
     // FIXME: putting this on the stack is risky.
     Value * vs[args[1]->listSize()];
@@ -2870,7 +2875,7 @@ static void prim_filter(EvalState & state, const PosIdx pos, Value * * args, Val
     for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
         Value res;
         state.callFunction(*args[0], *args[1]->listElems()[n], res, noPos);
-        if (state.forceBool(res, pos))
+        if (state.forceBool(res, pos, "while evaluating the return value of the filtering function passed to builtins.filter"))
             vs[k++] = args[1]->listElems()[n];
         else
             same = false;
@@ -2898,9 +2903,9 @@ static RegisterPrimOp primop_filter({
 static void prim_elem(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     bool res = false;
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.elem");
     for (auto elem : args[1]->listItems())
-        if (state.eqValues(*args[0], *elem)) {
+        if (state.eqValues(*args[0], *elem, pos, "while searching for the presence of the given element in the list")) {
             res = true;
             break;
         }
@@ -2920,8 +2925,8 @@ static RegisterPrimOp primop_elem({
 /* Concatenate a list of lists. */
 static void prim_concatLists(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
-    state.concatLists(v, args[0]->listSize(), args[0]->listElems(), pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.concatLists");
+    state.concatLists(v, args[0]->listSize(), args[0]->listElems(), pos, "while evaluating a value of the list passed to builtins.concatLists");
 }
 
 static RegisterPrimOp primop_concatLists({
@@ -2936,7 +2941,7 @@ static RegisterPrimOp primop_concatLists({
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.length");
     v.mkInt(args[0]->listSize());
 }
 
@@ -2953,8 +2958,8 @@ static RegisterPrimOp primop_length({
    right. The operator is applied strictly. */
 static void prim_foldlStrict(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[2], pos);
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.foldlStrict");
+    state.forceList(*args[2], pos, "while evaluating the third argument passed to builtins.foldlStrict");
 
     if (args[2]->listSize()) {
         Value * vCur = args[1];
@@ -2986,13 +2991,13 @@ static RegisterPrimOp primop_foldlStrict({
 
 static void anyOrAll(bool any, EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, std::string("while evaluating the first argument passed to builtins.") + (any ? "any" : "all"));
+    state.forceList(*args[1], pos, std::string("while evaluating the second argument passed to builtins.") + (any ? "any" : "all"));
 
     Value vTmp;
     for (auto elem : args[1]->listItems()) {
         state.callFunction(*args[0], *elem, vTmp, pos);
-        bool res = state.forceBool(vTmp, pos);
+        bool res = state.forceBool(vTmp, pos, std::string("while evaluating the return value of the function passed to builtins.") + (any ? "any" : "all"));
         if (res == any) {
             v.mkBool(any);
             return;
@@ -3035,16 +3040,16 @@ static RegisterPrimOp primop_all({
 
 static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto len = state.forceInt(*args[1], pos);
+    auto len = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.genList");
 
     if (len < 0)
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("cannot create list of size %1%", len),
-            .errPos = state.positions[pos]
-        }));
+        state.error("cannot create list of size %1%", len).debugThrow<EvalError>();
 
-    state.mkList(v, len);
+    // More strict than striclty (!) necessary, but acceptable
+    // as evaluating map without accessing any values makes little sense.
+    state.forceFunction(*args[0], noPos, "while evaluating the first argument passed to builtins.genList");
 
+    state.mkList(v, len);
     for (unsigned int n = 0; n < (unsigned int) len; ++n) {
         auto arg = state.allocValue();
         arg->mkInt(n);
@@ -3073,10 +3078,16 @@ static void prim_lessThan(EvalState & state, const PosIdx pos, Value * * args, V
 
 static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.sort");
 
     auto len = args[1]->listSize();
+    if (len == 0) {
+        v = *args[1];
+        return;
+    }
+
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.sort");
+
     state.mkList(v, len);
     for (unsigned int n = 0; n < len; ++n) {
         state.forceValue(*args[1]->listElems()[n], pos);
@@ -3086,13 +3097,15 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
     auto comparator = [&](Value * a, Value * b) {
         /* Optimization: if the comparator is lessThan, bypass
            callFunction. */
+        /* TODO: (layus) this is absurd. An optimisation like this
+           should be outside the lambda creation */
         if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
-            return CompareValues(state)(a, b);
+            return CompareValues(state, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
 
         Value * vs[] = {a, b};
         Value vBool;
-        state.callFunction(*args[0], 2, vs, vBool, pos);
-        return state.forceBool(vBool, pos);
+        state.callFunction(*args[0], 2, vs, vBool, noPos);
+        return state.forceBool(vBool, pos, "while evaluating the return value of the sorting function passed to builtins.sort");
     };
 
     /* FIXME: std::sort can segfault if the comparator is not a strict
@@ -3124,8 +3137,8 @@ static RegisterPrimOp primop_sort({
 
 static void prim_partition(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.partition");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.partition");
 
     auto len = args[1]->listSize();
 
@@ -3136,7 +3149,7 @@ static void prim_partition(EvalState & state, const PosIdx pos, Value * * args,
         state.forceValue(*vElem, pos);
         Value res;
         state.callFunction(*args[0], *vElem, res, pos);
-        if (state.forceBool(res, pos))
+        if (state.forceBool(res, pos, "while evaluating the return value of the partition function passed to builtins.partition"))
             right.push_back(vElem);
         else
             wrong.push_back(vElem);
@@ -3184,15 +3197,15 @@ static RegisterPrimOp primop_partition({
 
 static void prim_groupBy(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.groupBy");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.groupBy");
 
     ValueVectorMap attrs;
 
     for (auto vElem : args[1]->listItems()) {
         Value res;
         state.callFunction(*args[0], *vElem, res, pos);
-        auto name = state.forceStringNoCtx(res, pos);
+        auto name = state.forceStringNoCtx(res, pos, "while evaluating the return value of the grouping function passed to builtins.groupBy");
         auto sym = state.symbols.create(name);
         auto vector = attrs.try_emplace(sym, ValueVector()).first;
         vector->second.push_back(vElem);
@@ -3236,8 +3249,8 @@ static RegisterPrimOp primop_groupBy({
 
 static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.concatMap");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.concatMap");
     auto nrLists = args[1]->listSize();
 
     Value lists[nrLists];
@@ -3246,12 +3259,7 @@ static void prim_concatMap(EvalState & state, const PosIdx pos, Value * * args,
     for (unsigned int n = 0; n < nrLists; ++n) {
         Value * vElem = args[1]->listElems()[n];
         state.callFunction(*args[0], *vElem, lists[n], pos);
-        try {
-            state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)));
-        } catch (TypeError &e) {
-            e.addTrace(state.positions[pos], hintfmt("while invoking '%s'", "concatMap"));
-            state.debugThrowLastTrace(e);
-        }
+        state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)), "while evaluating the return value of the function passed to buitlins.concatMap");
         len += lists[n].listSize();
     }
 
@@ -3286,9 +3294,11 @@ static void prim_add(EvalState & state, const PosIdx pos, Value * * args, Value
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) + state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first argument of the addition")
+                + state.forceFloat(*args[1], pos, "while evaluating the second argument of the addition"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the addition")
+                + state.forceInt(*args[1], pos, "while evaluating the second argument of the addition"));
 }
 
 static RegisterPrimOp primop_add({
@@ -3305,9 +3315,11 @@ static void prim_sub(EvalState & state, const PosIdx pos, Value * * args, Value
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) - state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first argument of the subtraction")
+                - state.forceFloat(*args[1], pos, "while evaluating the second argument of the subtraction"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the subtraction")
+                - state.forceInt(*args[1], pos, "while evaluating the second argument of the subtraction"));
 }
 
 static RegisterPrimOp primop_sub({
@@ -3324,9 +3336,11 @@ static void prim_mul(EvalState & state, const PosIdx pos, Value * * args, Value
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) * state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first of the multiplication")
+                * state.forceFloat(*args[1], pos, "while evaluating the second argument of the multiplication"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the multiplication")
+                * state.forceInt(*args[1], pos, "while evaluating the second argument of the multiplication"));
 }
 
 static RegisterPrimOp primop_mul({
@@ -3343,7 +3357,7 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
 
-    NixFloat f2 = state.forceFloat(*args[1], pos);
+    NixFloat f2 = state.forceFloat(*args[1], pos, "while evaluating the second operand of the division");
     if (f2 == 0)
         state.debugThrowLastTrace(EvalError({
             .msg = hintfmt("division by zero"),
@@ -3351,10 +3365,10 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
         }));
 
     if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
-        v.mkFloat(state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first operand of the division") / f2);
     } else {
-        NixInt i1 = state.forceInt(*args[0], pos);
-        NixInt i2 = state.forceInt(*args[1], pos);
+        NixInt i1 = state.forceInt(*args[0], pos, "while evaluating the first operand of the division");
+        NixInt i2 = state.forceInt(*args[1], pos, "while evaluating the second operand of the division");
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
             state.debugThrowLastTrace(EvalError({
@@ -3377,7 +3391,8 @@ static RegisterPrimOp primop_div({
 
 static void prim_bitAnd(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) & state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitAnd")
+            & state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitAnd"));
 }
 
 static RegisterPrimOp primop_bitAnd({
@@ -3391,7 +3406,8 @@ static RegisterPrimOp primop_bitAnd({
 
 static void prim_bitOr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) | state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitOr")
+            | state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitOr"));
 }
 
 static RegisterPrimOp primop_bitOr({
@@ -3405,7 +3421,8 @@ static RegisterPrimOp primop_bitOr({
 
 static void prim_bitXor(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) ^ state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitXor")
+            ^ state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitXor"));
 }
 
 static RegisterPrimOp primop_bitXor({
@@ -3421,7 +3438,8 @@ static void prim_lessThan(EvalState & state, const PosIdx pos, Value * * args, V
 {
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
-    CompareValues comp{state};
+    // pos is exact here, no need for a message.
+    CompareValues comp(state, noPos, "");
     v.mkBool(comp(args[0], args[1]));
 }
 
@@ -3448,7 +3466,9 @@ static RegisterPrimOp primop_lessThan({
 static void prim_toString(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context, true, false);
+    auto s = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.toString",
+            true, false);
     v.mkString(*s, context);
 }
 
@@ -3482,10 +3502,10 @@ static RegisterPrimOp primop_toString({
    non-negative. */
 static void prim_substring(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    int start = state.forceInt(*args[0], pos);
-    int len = state.forceInt(*args[1], pos);
+    int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
+    int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
     PathSet context;
-    auto s = state.coerceToString(pos, *args[2], context);
+    auto s = state.coerceToString(pos, *args[2], context, "while evaluating the third argument (the string) passed to builtins.substring");
 
     if (start < 0)
         state.debugThrowLastTrace(EvalError({
@@ -3519,7 +3539,7 @@ static RegisterPrimOp primop_substring({
 static void prim_stringLength(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.stringLength");
     v.mkInt(s->size());
 }
 
@@ -3536,7 +3556,7 @@ static RegisterPrimOp primop_stringLength({
 /* Return the cryptographic hash of a string in base-16. */
 static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto type = state.forceStringNoCtx(*args[0], pos);
+    auto type = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.hashString");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
         state.debugThrowLastTrace(Error({
@@ -3545,7 +3565,7 @@ static void prim_hashString(EvalState & state, const PosIdx pos, Value * * args,
         }));
 
     PathSet context; // discarded
-    auto s = state.forceString(*args[1], context, pos);
+    auto s = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.hashString");
 
     v.mkString(hashString(*ht, s).to_string(Base16, false));
 }
@@ -3584,14 +3604,14 @@ std::shared_ptr<RegexCache> makeRegexCache()
 
 void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto re = state.forceStringNoCtx(*args[0], pos);
+    auto re = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.match");
 
     try {
 
         auto regex = state.regexCache->get(re);
 
         PathSet context;
-        const auto str = state.forceString(*args[1], context, pos);
+        const auto str = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.match");
 
         std::cmatch match;
         if (!std::regex_match(str.begin(), str.end(), match, regex)) {
@@ -3664,14 +3684,14 @@ static RegisterPrimOp primop_match({
    non-matching parts interleaved by the lists of the matching groups. */
 void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto re = state.forceStringNoCtx(*args[0], pos);
+    auto re = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.split");
 
     try {
 
         auto regex = state.regexCache->get(re);
 
         PathSet context;
-        const auto str = state.forceString(*args[1], context, pos);
+        const auto str = state.forceString(*args[1], context, pos, "while evaluating the second argument passed to builtins.split");
 
         auto begin = std::cregex_iterator(str.begin(), str.end(), regex);
         auto end = std::cregex_iterator();
@@ -3769,8 +3789,8 @@ static void prim_concatStringsSep(EvalState & state, const PosIdx pos, Value * *
 {
     PathSet context;
 
-    auto sep = state.forceString(*args[0], context, pos);
-    state.forceList(*args[1], pos);
+    auto sep = state.forceString(*args[0], context, pos, "while evaluating the first argument (the separator string) passed to builtins.concatStringsSep");
+    state.forceList(*args[1], pos, "while evaluating the second argument (the list of strings to concat) passed to builtins.concatStringsSep");
 
     std::string res;
     res.reserve((args[1]->listSize() + 32) * sep.size());
@@ -3778,7 +3798,7 @@ static void prim_concatStringsSep(EvalState & state, const PosIdx pos, Value * *
 
     for (auto elem : args[1]->listItems()) {
         if (first) first = false; else res += sep;
-        res += *state.coerceToString(pos, *elem, context);
+        res += *state.coerceToString(pos, *elem, context, "while evaluating one element of the list of strings to concat passed to builtins.concatStringsSep");
     }
 
     v.mkString(res, context);
@@ -3797,29 +3817,26 @@ static RegisterPrimOp primop_concatStringsSep({
 
 static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[0], pos, "while evaluating the first argument passed to builtins.replaceStrings");
+    state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.replaceStrings");
     if (args[0]->listSize() != args[1]->listSize())
-        state.debugThrowLastTrace(EvalError({
-            .msg = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
-            .errPos = state.positions[pos]
-        }));
+        state.error("'from' and 'to' arguments passed to builtins.replaceStrings have different lengths").atPos(pos).debugThrow<EvalError>();
 
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
     for (auto elem : args[0]->listItems())
-        from.emplace_back(state.forceString(*elem, pos));
+        from.emplace_back(state.forceString(*elem, pos, "while evaluating one of the strings to replace passed to builtins.replaceStrings"));
 
     std::vector<std::pair<std::string, PathSet>> to;
     to.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
         PathSet ctx;
-        auto s = state.forceString(*elem, ctx, pos);
+        auto s = state.forceString(*elem, ctx, pos, "while evaluating one of the replacement strings passed to builtins.replaceStrings");
         to.emplace_back(s, std::move(ctx));
     }
 
     PathSet context;
-    auto s = state.forceString(*args[2], context, pos);
+    auto s = state.forceString(*args[2], context, pos, "while evaluating the third argument passed to builtins.replaceStrings");
 
     std::string res;
     // Loops one past last character to handle the case where 'from' contains an empty string.
@@ -3877,7 +3894,7 @@ static RegisterPrimOp primop_replaceStrings({
 
 static void prim_parseDrvName(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto name = state.forceStringNoCtx(*args[0], pos);
+    auto name = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.parseDrvName");
     DrvName parsed(name);
     auto attrs = state.buildBindings(2);
     attrs.alloc(state.sName).mkString(parsed.name);
@@ -3901,8 +3918,8 @@ static RegisterPrimOp primop_parseDrvName({
 
 static void prim_compareVersions(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto version1 = state.forceStringNoCtx(*args[0], pos);
-    auto version2 = state.forceStringNoCtx(*args[1], pos);
+    auto version1 = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.compareVersions");
+    auto version2 = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.compareVersions");
     v.mkInt(compareVersions(version1, version2));
 }
 
@@ -3921,7 +3938,7 @@ static RegisterPrimOp primop_compareVersions({
 
 static void prim_splitVersion(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto version = state.forceStringNoCtx(*args[0], pos);
+    auto version = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.splitVersion");
     auto iter = version.cbegin();
     Strings components;
     while (iter != version.cend()) {
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 4b7357495..0c65a6b98 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -8,7 +8,7 @@ namespace nix {
 static void prim_unsafeDiscardStringContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardStringContext");
     v.mkString(*s);
 }
 
@@ -18,7 +18,7 @@ static RegisterPrimOp primop_unsafeDiscardStringContext("__unsafeDiscardStringCo
 static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.hasContext");
     v.mkBool(!context.empty());
 }
 
@@ -34,7 +34,7 @@ static RegisterPrimOp primop_hasContext("__hasContext", 1, prim_hasContext);
 static void prim_unsafeDiscardOutputDependency(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto s = state.coerceToString(pos, *args[0], context);
+    auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.unsafeDiscardOutputDependency");
 
     PathSet context2;
     for (auto && p : context) {
@@ -80,7 +80,7 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
         Strings outputs;
     };
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "while evaluating the argument passed to builtins.getContext");
     auto contextInfos = std::map<StorePath, ContextInfo>();
     for (const auto & p : context) {
         Path drv;
@@ -132,9 +132,9 @@ static RegisterPrimOp primop_getContext("__getContext", 1, prim_getContext);
 static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto orig = state.forceString(*args[0], context, pos);
+    auto orig = state.forceString(*args[0], context, noPos, "while evaluating the first argument passed to builtins.appendContext");
 
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[1], pos, "while evaluating the second argument passed to builtins.appendContext");
 
     auto sPath = state.symbols.create("path");
     auto sAllOutputs = state.symbols.create("allOutputs");
@@ -142,24 +142,24 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
         const auto & name = state.symbols[i.name];
         if (!state.store->isStorePath(name))
             throw EvalError({
-                .msg = hintfmt("Context key '%s' is not a store path", name),
+                .msg = hintfmt("context key '%s' is not a store path", name),
                 .errPos = state.positions[i.pos]
             });
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(name));
-        state.forceAttrs(*i.value, i.pos);
+        state.forceAttrs(*i.value, i.pos, "while evaluating the value of a string context");
         auto iter = i.value->attrs->find(sPath);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, iter->pos))
+            if (state.forceBool(*iter->value, iter->pos, "while evaluating the `path` attribute of a string context"))
                 context.emplace(name);
         }
 
         iter = i.value->attrs->find(sAllOutputs);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, iter->pos)) {
+            if (state.forceBool(*iter->value, iter->pos, "while evaluating the `allOutputs` attribute of a string context")) {
                 if (!isDerivation(name)) {
                     throw EvalError({
-                        .msg = hintfmt("Tried to add all-outputs context of %s, which is not a derivation, to a string", name),
+                        .msg = hintfmt("tried to add all-outputs context of %s, which is not a derivation, to a string", name),
                         .errPos = state.positions[i.pos]
                     });
                 }
@@ -169,15 +169,15 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
 
         iter = i.value->attrs->find(state.sOutputs);
         if (iter != i.value->attrs->end()) {
-            state.forceList(*iter->value, iter->pos);
+            state.forceList(*iter->value, iter->pos, "while evaluating the `outputs` attribute of a string context");
             if (iter->value->listSize() && !isDerivation(name)) {
                 throw EvalError({
-                    .msg = hintfmt("Tried to add derivation output context of %s, which is not a derivation, to a string", name),
+                    .msg = hintfmt("tried to add derivation output context of %s, which is not a derivation, to a string", name),
                     .errPos = state.positions[i.pos]
                 });
             }
             for (auto elem : iter->value->listItems()) {
-                auto outputName = state.forceStringNoCtx(*elem, iter->pos);
+                auto outputName = state.forceStringNoCtx(*elem, iter->pos, "while evaluating an output name within a string context");
                 context.insert(concatStrings("!", outputName, "!", name));
             }
         }
diff --git a/src/libexpr/primops/fetchClosure.cc b/src/libexpr/primops/fetchClosure.cc
index 662c9652e..0dfa97fa3 100644
--- a/src/libexpr/primops/fetchClosure.cc
+++ b/src/libexpr/primops/fetchClosure.cc
@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.fetchClosure");
 
     std::optional<std::string> fromStoreUrl;
     std::optional<StorePath> fromPath;
@@ -19,7 +19,8 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
 
         if (attrName == "fromPath") {
             PathSet context;
-            fromPath = state.coerceToStorePath(attr.pos, *attr.value, context);
+            fromPath = state.coerceToStorePath(attr.pos, *attr.value, context,
+                    "while evaluating the 'fromPath' attribute passed to builtins.fetchClosure");
         }
 
         else if (attrName == "toPath") {
@@ -27,12 +28,14 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
             toCA = true;
             if (attr.value->type() != nString || attr.value->string.s != std::string("")) {
                 PathSet context;
-                toPath = state.coerceToStorePath(attr.pos, *attr.value, context);
+                toPath = state.coerceToStorePath(attr.pos, *attr.value, context,
+                        "while evaluating the 'toPath' attribute passed to builtins.fetchClosure");
             }
         }
 
         else if (attrName == "fromStore")
-            fromStoreUrl = state.forceStringNoCtx(*attr.value, attr.pos);
+            fromStoreUrl = state.forceStringNoCtx(*attr.value, attr.pos,
+                    "while evaluating the 'fromStore' attribute passed to builtins.fetchClosure");
 
         else
             throw Error({
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index 249c0934e..c41bd60b6 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -19,23 +19,23 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
-                url = state.coerceToString(attr.pos, *attr.value, context, false, false).toOwned();
+                url = state.coerceToString(attr.pos, *attr.value, context,
+                        "while evaluating the `url` attribute passed to builtins.fetchMercurial",
+                        false, false).toOwned();
             else if (n == "rev") {
                 // Ugly: unlike fetchGit, here the "rev" attribute can
                 // be both a revision or a branch/tag name.
-                auto value = state.forceStringNoCtx(*attr.value, attr.pos);
+                auto value = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `rev` attribute passed to builtins.fetchMercurial");
                 if (std::regex_match(value.begin(), value.end(), revRegex))
                     rev = Hash::parseAny(value, htSHA1);
                 else
                     ref = value;
             }
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, attr.pos);
+                name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `name` attribute passed to builtins.fetchMercurial");
             else
                 throw EvalError({
                     .msg = hintfmt("unsupported argument '%s' to 'fetchMercurial'", state.symbols[attr.name]),
@@ -50,7 +50,9 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             });
 
     } else
-        url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        url = state.coerceToString(pos, *args[0], context,
+                "while evaluating the first argument passed to builtins.fetchMercurial",
+                false, false).toOwned();
 
     // FIXME: git externals probably can be used to bypass the URI
     // whitelist. Ah well.
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index fb392a6e8..83d93b75c 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -102,7 +102,7 @@ static void fetchTree(
     state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
-        state.forceAttrs(*args[0], pos);
+        state.forceAttrs(*args[0], pos, "while evaluating the argument passed to builtins.fetchTree");
 
         fetchers::Attrs attrs;
 
@@ -112,7 +112,7 @@ static void fetchTree(
                     .msg = hintfmt("unexpected attribute 'type'"),
                     .errPos = state.positions[pos]
                 }));
-            type = state.forceStringNoCtx(*aType->value, aType->pos);
+            type = state.forceStringNoCtx(*aType->value, aType->pos, "while evaluating the `type` attribute passed to builtins.fetchTree");
         } else if (!type)
             state.debugThrowLastTrace(EvalError({
                 .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
@@ -125,7 +125,7 @@ static void fetchTree(
             if (attr.name == state.sType) continue;
             state.forceValue(*attr.value, attr.pos);
             if (attr.value->type() == nPath || attr.value->type() == nString) {
-                auto s = state.coerceToString(attr.pos, *attr.value, context, false, false).toOwned();
+                auto s = state.coerceToString(attr.pos, *attr.value, context, "", false, false).toOwned();
                 attrs.emplace(state.symbols[attr.name],
                     state.symbols[attr.name] == "url"
                     ? type == "git"
@@ -151,7 +151,9 @@ static void fetchTree(
 
         input = fetchers::Input::fromAttrs(std::move(attrs));
     } else {
-        auto url = state.coerceToString(pos, *args[0], context, false, false).toOwned();
+        auto url = state.coerceToString(pos, *args[0], context,
+                "while evaluating the first argument passed to the fetcher",
+                false, false).toOwned();
 
         if (type == "git") {
             fetchers::Attrs attrs;
@@ -195,16 +197,14 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(state.symbols[attr.name]);
             if (n == "url")
-                url = state.forceStringNoCtx(*attr.value, attr.pos);
+                url = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the url we should fetch");
             else if (n == "sha256")
-                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos), htSHA256);
+                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the sha256 of the content we should fetch"), htSHA256);
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, attr.pos);
+                name = state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the name of the content we should fetch");
             else
                 state.debugThrowLastTrace(EvalError({
                     .msg = hintfmt("unsupported argument '%s' to '%s'", n, who),
@@ -218,7 +218,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
                 .errPos = state.positions[pos]
             }));
     } else
-        url = state.forceStringNoCtx(*args[0], pos);
+        url = state.forceStringNoCtx(*args[0], pos, "while evaluating the url we should fetch");
 
     if (who == "fetchTarball")
         url = evalSettings.resolvePseudoUrl(*url);
diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index 9753e2ac9..8a5231781 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, Value & val)
 {
-    auto toml = state.forceStringNoCtx(*args[0], pos);
+    auto toml = state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.fromTOML");
 
     std::istringstream tomlStream(std::string{toml});
 
diff --git a/src/libexpr/tests/error_traces.cc b/src/libexpr/tests/error_traces.cc
new file mode 100644
index 000000000..5e2213f69
--- /dev/null
+++ b/src/libexpr/tests/error_traces.cc
@@ -0,0 +1,1298 @@
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "libexprtests.hh"
+
+namespace nix {
+
+    using namespace testing;
+
+    // Testing eval of PrimOp's
+    class ErrorTraceTest : public LibExprTest { };
+
+    TEST_F(ErrorTraceTest, TraceBuilder) {
+        ASSERT_THROW(
+            state.error("Not much").debugThrow<EvalError>(),
+            EvalError
+        );
+
+        ASSERT_THROW(
+            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>(),
+            EvalError
+        );
+
+        ASSERT_THROW(
+            try {
+                try {
+                    state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+                } catch (Error & e) {
+                    e.addTrace(state.positions[noPos], "Something", "");
+                    throw;
+                }
+            } catch (BaseError & e) {
+                ASSERT_EQ(PrintToString(e.info().msg),
+                          PrintToString(hintfmt("Not much")));
+                auto trace = e.info().traces.rbegin();
+                ASSERT_EQ(e.info().traces.size(), 2);
+                ASSERT_EQ(PrintToString(trace->hint),
+                          PrintToString(hintfmt("No more")));
+                trace++;
+                ASSERT_EQ(PrintToString(trace->hint),
+                          PrintToString(hintfmt("Something")));
+                throw;
+            }
+            , EvalError
+        );
+    }
+
+    TEST_F(ErrorTraceTest, NestedThrows) {
+        try {
+            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+        } catch (BaseError & e) {
+            try {
+                state.error("Not much more").debugThrow<EvalError>();
+            } catch (Error & e2) {
+                e.addTrace(state.positions[noPos], "Something", "");
+                //e2.addTrace(state.positions[noPos], "Something", "");
+                ASSERT_TRUE(e.info().traces.size() == 2);
+                ASSERT_TRUE(e2.info().traces.size() == 0);
+                ASSERT_FALSE(&e.info() == &e2.info());
+            }
+        }
+    }
+
+#define ASSERT_TRACE1(args, type, message)                                  \
+        ASSERT_THROW(                                                       \
+            std::string expr(args);                                         \
+            std::string name = expr.substr(0, expr.find(" "));              \
+            try {                                                           \
+                Value v = eval("builtins." args);                           \
+                state.forceValueDeep(v);                                    \
+            } catch (BaseError & e) {                                       \
+                ASSERT_EQ(PrintToString(e.info().msg),                      \
+                          PrintToString(message));                          \
+                ASSERT_EQ(e.info().traces.size(), 1) << "while testing " args << std::endl << e.what(); \
+                auto trace = e.info().traces.rbegin();                      \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(hintfmt("while calling the '%s' builtin", name))); \
+                throw;                                                      \
+            }                                                               \
+            , type                                                          \
+        )
+
+#define ASSERT_TRACE2(args, type, message, context)                         \
+        ASSERT_THROW(                                                       \
+            std::string expr(args);                                         \
+            std::string name = expr.substr(0, expr.find(" "));              \
+            try {                                                           \
+                Value v = eval("builtins." args);                           \
+                state.forceValueDeep(v);                                    \
+            } catch (BaseError & e) {                                       \
+                ASSERT_EQ(PrintToString(e.info().msg),                      \
+                          PrintToString(message));                          \
+                ASSERT_EQ(e.info().traces.size(), 2) << "while testing " args << std::endl << e.what(); \
+                auto trace = e.info().traces.rbegin();                      \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(context));                          \
+                ++trace;                                                    \
+                ASSERT_EQ(PrintToString(trace->hint),                       \
+                          PrintToString(hintfmt("while calling the '%s' builtin", name))); \
+                throw;                                                      \
+            }                                                               \
+            , type                                                          \
+        )
+
+    TEST_F(ErrorTraceTest, genericClosure) {
+        ASSERT_TRACE2("genericClosure 1",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure {}",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "startSet"),
+                      hintfmt("in the attrset passed as argument to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = 1; }",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the 'startSet' attribute passed as argument to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = true; }",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "a Boolean"),
+                      hintfmt("while evaluating the 'operator' attribute passed as argument to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: true; }",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a Boolean"),
+                      hintfmt("while evaluating the return value of the `operator` passed to builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [ true ]; }",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a Boolean"),
+                      hintfmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "key"),
+                      hintfmt("in one of the attrsets generated by (or initially passed to) builtins.genericClosure"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [{ key = ''a''; }]; }",
+                      EvalError,
+                      hintfmt("cannot compare %s with %s", "a string", "an integer"),
+                      hintfmt("while comparing the `key` attributes of two genericClosure elements"));
+
+        ASSERT_TRACE2("genericClosure { startSet = [ true ]; operator = item: [{ key = ''a''; }]; }",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a Boolean"),
+                      hintfmt("while evaluating one of the elements generated by (or initially passed to) builtins.genericClosure"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, replaceStrings) {
+        ASSERT_TRACE2("replaceStrings 0 0 {}",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.replaceStrings"));
+
+        ASSERT_TRACE2("replaceStrings [] 0 {}",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the second argument passed to builtins.replaceStrings"));
+
+        ASSERT_TRACE1("replaceStrings [ 0 ] [] {}",
+                      EvalError,
+                      hintfmt("'from' and 'to' arguments passed to builtins.replaceStrings have different lengths"));
+
+        ASSERT_TRACE2("replaceStrings [ 1 ] [ \"new\" ] {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating one of the strings to replace passed to builtins.replaceStrings"));
+
+        ASSERT_TRACE2("replaceStrings [ \"old\" ] [ true ] {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a Boolean"),
+                      hintfmt("while evaluating one of the replacement strings passed to builtins.replaceStrings"));
+
+        ASSERT_TRACE2("replaceStrings [ \"old\" ] [ \"new\" ] {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a set"),
+                      hintfmt("while evaluating the third argument passed to builtins.replaceStrings"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, scopedImport) {
+    }
+
+
+    TEST_F(ErrorTraceTest, import) {
+    }
+
+
+    TEST_F(ErrorTraceTest, typeOf) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isNull) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isFunction) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isInt) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isFloat) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isString) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isBool) {
+    }
+
+
+    TEST_F(ErrorTraceTest, isPath) {
+    }
+
+
+    TEST_F(ErrorTraceTest, break) {
+    }
+
+
+    TEST_F(ErrorTraceTest, abort) {
+    }
+
+
+    TEST_F(ErrorTraceTest, throw) {
+    }
+
+
+    TEST_F(ErrorTraceTest, addErrorContext) {
+    }
+
+
+    TEST_F(ErrorTraceTest, ceil) {
+        ASSERT_TRACE2("ceil \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a float was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.ceil"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, floor) {
+        ASSERT_TRACE2("floor \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a float was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.floor"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, tryEval) {
+    }
+
+
+    TEST_F(ErrorTraceTest, getEnv) {
+        ASSERT_TRACE2("getEnv [ ]",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.getEnv"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, seq) {
+    }
+
+
+    TEST_F(ErrorTraceTest, deepSeq) {
+    }
+
+
+    TEST_F(ErrorTraceTest, trace) {
+    }
+
+
+    TEST_F(ErrorTraceTest, placeholder) {
+        ASSERT_TRACE2("placeholder []",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.placeholder"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, toPath) {
+        ASSERT_TRACE2("toPath []",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.toPath"));
+
+        ASSERT_TRACE2("toPath \"foo\"",
+                      EvalError,
+                      hintfmt("string '%s' doesn't represent an absolute path", "foo"),
+                      hintfmt("while evaluating the first argument passed to builtins.toPath"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, storePath) {
+        ASSERT_TRACE2("storePath true",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a Boolean"),
+                      hintfmt("while evaluating the first argument passed to builtins.storePath"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, pathExists) {
+        ASSERT_TRACE2("pathExists []",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("while realising the context of a path"));
+
+        ASSERT_TRACE2("pathExists \"zorglub\"",
+                      EvalError,
+                      hintfmt("string '%s' doesn't represent an absolute path", "zorglub"),
+                      hintfmt("while realising the context of a path"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, baseNameOf) {
+        ASSERT_TRACE2("baseNameOf []",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.baseNameOf"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, dirOf) {
+    }
+
+
+    TEST_F(ErrorTraceTest, readFile) {
+    }
+
+
+    TEST_F(ErrorTraceTest, findFile) {
+    }
+
+
+    TEST_F(ErrorTraceTest, hashFile) {
+    }
+
+
+    TEST_F(ErrorTraceTest, readDir) {
+    }
+
+
+    TEST_F(ErrorTraceTest, toXML) {
+    }
+
+
+    TEST_F(ErrorTraceTest, toJSON) {
+    }
+
+
+    TEST_F(ErrorTraceTest, fromJSON) {
+    }
+
+
+    TEST_F(ErrorTraceTest, toFile) {
+    }
+
+
+    TEST_F(ErrorTraceTest, filterSource) {
+        ASSERT_TRACE2("filterSource [] []",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("while evaluating the second argument (the path to filter) passed to builtins.filterSource"));
+
+        ASSERT_TRACE2("filterSource [] \"foo\"",
+                      EvalError,
+                      hintfmt("string '%s' doesn't represent an absolute path", "foo"),
+                      hintfmt("while evaluating the second argument (the path to filter) passed to builtins.filterSource"));
+
+        ASSERT_TRACE2("filterSource [] ./.",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.filterSource"));
+
+        // Usupported by store "dummy"
+
+        // ASSERT_TRACE2("filterSource (_: 1) ./.",
+        //               TypeError,
+        //               hintfmt("attempt to call something which is not a function but %s", "an integer"),
+        //               hintfmt("while adding path '/home/layus/projects/nix'"));
+
+        // ASSERT_TRACE2("filterSource (_: _: 1) ./.",
+        //               TypeError,
+        //               hintfmt("value is %s while a Boolean was expected", "an integer"),
+        //               hintfmt("while evaluating the return value of the path filter function"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, path) {
+    }
+
+
+    TEST_F(ErrorTraceTest, attrNames) {
+        ASSERT_TRACE2("attrNames []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the argument passed to builtins.attrNames"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, attrValues) {
+        ASSERT_TRACE2("attrValues []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the argument passed to builtins.attrValues"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, getAttr) {
+        ASSERT_TRACE2("getAttr [] []",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.getAttr"));
+
+        ASSERT_TRACE2("getAttr \"foo\" []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the second argument passed to builtins.getAttr"));
+
+        ASSERT_TRACE2("getAttr \"foo\" {}",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "foo"),
+                      hintfmt("in the attribute set under consideration"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, unsafeGetAttrPos) {
+    }
+
+
+    TEST_F(ErrorTraceTest, hasAttr) {
+        ASSERT_TRACE2("hasAttr [] []",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.hasAttr"));
+
+        ASSERT_TRACE2("hasAttr \"foo\" []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the second argument passed to builtins.hasAttr"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, isAttrs) {
+    }
+
+
+    TEST_F(ErrorTraceTest, removeAttrs) {
+        ASSERT_TRACE2("removeAttrs \"\" \"\"",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.removeAttrs"));
+
+        ASSERT_TRACE2("removeAttrs \"\" [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.removeAttrs"));
+
+        ASSERT_TRACE2("removeAttrs \"\" [ \"1\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.removeAttrs"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, listToAttrs) {
+        ASSERT_TRACE2("listToAttrs 1",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the argument passed to builtins.listToAttrs"));
+
+        ASSERT_TRACE2("listToAttrs [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating an element of the list passed to builtins.listToAttrs"));
+
+        ASSERT_TRACE2("listToAttrs [ {} ]",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "name"),
+                      hintfmt("in a {name=...; value=...;} pair"));
+
+        ASSERT_TRACE2("listToAttrs [ { name = 1; } ]",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs"));
+
+        ASSERT_TRACE2("listToAttrs [ { name = \"foo\"; } ]",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "value"),
+                      hintfmt("in a {name=...; value=...;} pair"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, intersectAttrs) {
+        ASSERT_TRACE2("intersectAttrs [] []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.intersectAttrs"));
+
+        ASSERT_TRACE2("intersectAttrs {} []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the second argument passed to builtins.intersectAttrs"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, catAttrs) {
+        ASSERT_TRACE2("catAttrs [] {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.catAttrs"));
+
+        ASSERT_TRACE2("catAttrs \"foo\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a set"),
+                      hintfmt("while evaluating the second argument passed to builtins.catAttrs"));
+
+        ASSERT_TRACE2("catAttrs \"foo\" [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating an element in the list passed as second argument to builtins.catAttrs"));
+
+        ASSERT_TRACE2("catAttrs \"foo\" [ { foo = 1; } 1 { bar = 5;} ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating an element in the list passed as second argument to builtins.catAttrs"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, functionArgs) {
+        ASSERT_TRACE1("functionArgs {}",
+                      TypeError,
+                      hintfmt("'functionArgs' requires a function"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, mapAttrs) {
+        ASSERT_TRACE2("mapAttrs [] []",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a list"),
+                      hintfmt("while evaluating the second argument passed to builtins.mapAttrs"));
+
+        // XXX: defered
+        // ASSERT_TRACE2("mapAttrs \"\" { foo.bar = 1; }",
+        //               TypeError,
+        //               hintfmt("attempt to call something which is not a function but %s", "a string"),
+        //               hintfmt("while evaluating the attribute 'foo'"));
+
+        // ASSERT_TRACE2("mapAttrs (x: x + \"1\") { foo.bar = 1; }",
+        //               TypeError,
+        //               hintfmt("attempt to call something which is not a function but %s", "a string"),
+        //               hintfmt("while evaluating the attribute 'foo'"));
+
+        // ASSERT_TRACE2("mapAttrs (x: y: x + 1) { foo.bar = 1; }",
+        //               TypeError,
+        //               hintfmt("cannot coerce %s to a string", "an integer"),
+        //               hintfmt("while evaluating a path segment"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, zipAttrsWith) {
+        ASSERT_TRACE2("zipAttrsWith [] [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "a list"),
+                      hintfmt("while evaluating the first argument passed to builtins.zipAttrsWith"));
+
+        ASSERT_TRACE2("zipAttrsWith (_: 1) [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "an integer"),
+                      hintfmt("while evaluating a value of the list passed as second argument to builtins.zipAttrsWith"));
+
+        // XXX: How to properly tell that the fucntion takes two arguments ?
+        // The same question also applies to sort, and maybe others.
+        // Due to lazyness, we only create a thunk, and it fails later on.
+        // ASSERT_TRACE2("zipAttrsWith (_: 1) [ { foo = 1; } ]",
+        //               TypeError,
+        //               hintfmt("attempt to call something which is not a function but %s", "an integer"),
+        //               hintfmt("while evaluating the attribute 'foo'"));
+
+        // XXX: Also deferred deeply
+        // ASSERT_TRACE2("zipAttrsWith (a: b: a + b) [ { foo = 1; } { foo = 2; } ]",
+        //               TypeError,
+        //               hintfmt("cannot coerce %s to a string", "a list"),
+        //               hintfmt("while evaluating a path segment"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, isList) {
+    }
+
+
+    TEST_F(ErrorTraceTest, elemAt) {
+        ASSERT_TRACE2("elemAt \"foo\" (-1)",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.elemAt"));
+
+        ASSERT_TRACE1("elemAt [] (-1)",
+                      Error,
+                      hintfmt("list index %d is out of bounds", -1));
+
+        ASSERT_TRACE1("elemAt [\"foo\"] 3",
+                      Error,
+                      hintfmt("list index %d is out of bounds", 3));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, head) {
+        ASSERT_TRACE2("head 1",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.elemAt"));
+
+        ASSERT_TRACE1("head []",
+                      Error,
+                      hintfmt("list index %d is out of bounds", 0));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, tail) {
+        ASSERT_TRACE2("tail 1",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.tail"));
+
+        ASSERT_TRACE1("tail []",
+                      Error,
+                      hintfmt("'tail' called on an empty list"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, map) {
+        ASSERT_TRACE2("map 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.map"));
+
+        ASSERT_TRACE2("map 1 [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.map"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, filter) {
+        ASSERT_TRACE2("filter 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.filter"));
+
+        ASSERT_TRACE2("filter 1 [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.filter"));
+
+        ASSERT_TRACE2("filter (_: 5) [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the filtering function passed to builtins.filter"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, elem) {
+        ASSERT_TRACE2("elem 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.elem"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, concatLists) {
+        ASSERT_TRACE2("concatLists 1",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.concatLists"));
+
+        ASSERT_TRACE2("concatLists [ 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating a value of the list passed to builtins.concatLists"));
+
+        ASSERT_TRACE2("concatLists [ [1] \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating a value of the list passed to builtins.concatLists"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, length) {
+        ASSERT_TRACE2("length 1",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.length"));
+
+        ASSERT_TRACE2("length \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the first argument passed to builtins.length"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, foldlPrime) {
+        ASSERT_TRACE2("foldl' 1 \"foo\" true",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.foldlStrict"));
+
+        ASSERT_TRACE2("foldl' (_: 1) \"foo\" true",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a Boolean"),
+                      hintfmt("while evaluating the third argument passed to builtins.foldlStrict"));
+
+        ASSERT_TRACE1("foldl' (_: 1) \"foo\" [ true ]",
+                      TypeError,
+                      hintfmt("attempt to call something which is not a function but %s", "an integer"));
+
+        ASSERT_TRACE2("foldl' (a: b: a && b) \"foo\" [ true ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "a string"),
+                      hintfmt("in the left operand of the AND (&&) operator"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, any) {
+        ASSERT_TRACE2("any 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.any"));
+
+        ASSERT_TRACE2("any (_: 1) \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.any"));
+
+        ASSERT_TRACE2("any (_: 1) [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the function passed to builtins.any"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, all) {
+        ASSERT_TRACE2("all 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.all"));
+
+        ASSERT_TRACE2("all (_: 1) \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.all"));
+
+        ASSERT_TRACE2("all (_: 1) [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the function passed to builtins.all"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, genList) {
+        ASSERT_TRACE2("genList 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.genList"));
+
+        ASSERT_TRACE2("genList 1 2",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.genList", "an integer"));
+
+        // XXX: defered
+        // ASSERT_TRACE2("genList (x: x + \"foo\") 2 #TODO",
+        //               TypeError,
+        //               hintfmt("cannot add %s to an integer", "a string"),
+        //               hintfmt("while evaluating anonymous lambda"));
+
+        ASSERT_TRACE1("genList false (-3)",
+                      EvalError,
+                      hintfmt("cannot create list of size %d", -3));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, sort) {
+        ASSERT_TRACE2("sort 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.sort"));
+
+        ASSERT_TRACE2("sort 1 [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.sort"));
+
+        ASSERT_TRACE1("sort (_: 1) [ \"foo\" \"bar\" ]",
+                      TypeError,
+                      hintfmt("attempt to call something which is not a function but %s", "an integer"));
+
+        ASSERT_TRACE2("sort (_: _: 1) [ \"foo\" \"bar\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the sorting function passed to builtins.sort"));
+
+        // XXX: Trace too deep, need better asserts
+        // ASSERT_TRACE1("sort (a: b: a <= b) [ \"foo\" {} ] # TODO",
+        //               TypeError,
+        //               hintfmt("cannot compare %s with %s", "a string", "a set"));
+
+        // ASSERT_TRACE1("sort (a: b: a <= b) [ {} {} ] # TODO",
+        //               TypeError,
+        //               hintfmt("cannot compare %s with %s; values of that type are incomparable", "a set", "a set"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, partition) {
+        ASSERT_TRACE2("partition 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.partition"));
+
+        ASSERT_TRACE2("partition (_: 1) \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.partition"));
+
+        ASSERT_TRACE2("partition (_: 1) [ \"foo\" ]",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the partition function passed to builtins.partition"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, groupBy) {
+        ASSERT_TRACE2("groupBy 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.groupBy"));
+
+        ASSERT_TRACE2("groupBy (_: 1) \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.groupBy"));
+
+        ASSERT_TRACE2("groupBy (x: x) [ \"foo\" \"bar\" 1 ]",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the grouping function passed to builtins.groupBy"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, concatMap) {
+        ASSERT_TRACE2("concatMap 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a function was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.concatMap"));
+
+        ASSERT_TRACE2("concatMap (x: 1) \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the second argument passed to builtins.concatMap"));
+
+        ASSERT_TRACE2("concatMap (x: 1) [ \"foo\" ] # TODO",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "an integer"),
+                      hintfmt("while evaluating the return value of the function passed to buitlins.concatMap"));
+
+        ASSERT_TRACE2("concatMap (x: \"foo\") [ 1 2 ] # TODO",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the return value of the function passed to buitlins.concatMap"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, add) {
+        ASSERT_TRACE2("add \"foo\" 1",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the first argument of the addition"));
+
+        ASSERT_TRACE2("add 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the second argument of the addition"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, sub) {
+        ASSERT_TRACE2("sub \"foo\" 1",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the first argument of the subtraction"));
+
+        ASSERT_TRACE2("sub 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the second argument of the subtraction"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, mul) {
+        ASSERT_TRACE2("mul \"foo\" 1",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the first argument of the multiplication"));
+
+        ASSERT_TRACE2("mul 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the second argument of the multiplication"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, div) {
+        ASSERT_TRACE2("div \"foo\" 1 # TODO: an integer was expected -> a number",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the first operand of the division"));
+
+        ASSERT_TRACE2("div 1 \"foo\"",
+                      TypeError,
+                      hintfmt("value is %s while a float was expected", "a string"),
+                      hintfmt("while evaluating the second operand of the division"));
+
+        ASSERT_TRACE1("div \"foo\" 0",
+                      EvalError,
+                      hintfmt("division by zero"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, bitAnd) {
+        ASSERT_TRACE2("bitAnd 1.1 2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the first argument passed to builtins.bitAnd"));
+
+        ASSERT_TRACE2("bitAnd 1 2.2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the second argument passed to builtins.bitAnd"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, bitOr) {
+        ASSERT_TRACE2("bitOr 1.1 2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the first argument passed to builtins.bitOr"));
+
+        ASSERT_TRACE2("bitOr 1 2.2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the second argument passed to builtins.bitOr"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, bitXor) {
+        ASSERT_TRACE2("bitXor 1.1 2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the first argument passed to builtins.bitXor"));
+
+        ASSERT_TRACE2("bitXor 1 2.2",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a float"),
+                      hintfmt("while evaluating the second argument passed to builtins.bitXor"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, lessThan) {
+        ASSERT_TRACE1("lessThan 1 \"foo\"",
+                      EvalError,
+                      hintfmt("cannot compare %s with %s", "an integer", "a string"));
+
+        ASSERT_TRACE1("lessThan {} {}",
+                      EvalError,
+                      hintfmt("cannot compare %s with %s; values of that type are incomparable", "a set", "a set"));
+
+        ASSERT_TRACE2("lessThan [ 1 2 ] [ \"foo\" ]",
+                      EvalError,
+                      hintfmt("cannot compare %s with %s", "an integer", "a string"),
+                      hintfmt("while comparing two list elements"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, toString) {
+        ASSERT_TRACE2("toString { a = 1; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the first argument passed to builtins.toString"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, substring) {
+        ASSERT_TRACE2("substring {} \"foo\" true",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a set"),
+                      hintfmt("while evaluating the first argument (the start offset) passed to builtins.substring"));
+
+        ASSERT_TRACE2("substring 3 \"foo\" true",
+                      TypeError,
+                      hintfmt("value is %s while an integer was expected", "a string"),
+                      hintfmt("while evaluating the second argument (the substring length) passed to builtins.substring"));
+
+        ASSERT_TRACE2("substring 0 3 {}",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the third argument (the string) passed to builtins.substring"));
+
+        ASSERT_TRACE1("substring (-3) 3 \"sometext\"",
+                      EvalError,
+                      hintfmt("negative start position in 'substring'"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, stringLength) {
+        ASSERT_TRACE2("stringLength {} # TODO: context is missing ???",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the argument passed to builtins.stringLength"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, hashString) {
+        ASSERT_TRACE2("hashString 1 {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.hashString"));
+
+        ASSERT_TRACE1("hashString \"foo\" \"content\"",
+                      UsageError,
+                      hintfmt("unknown hash algorithm '%s'", "foo"));
+
+        ASSERT_TRACE2("hashString \"sha256\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a set"),
+                      hintfmt("while evaluating the second argument passed to builtins.hashString"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, match) {
+        ASSERT_TRACE2("match 1 {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.match"));
+
+        ASSERT_TRACE2("match \"foo\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a set"),
+                      hintfmt("while evaluating the second argument passed to builtins.match"));
+
+        ASSERT_TRACE1("match \"(.*\" \"\"",
+                      EvalError,
+                      hintfmt("invalid regular expression '%s'", "(.*"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, split) {
+        ASSERT_TRACE2("split 1 {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.split"));
+
+        ASSERT_TRACE2("split \"foo\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a set"),
+                      hintfmt("while evaluating the second argument passed to builtins.split"));
+
+        ASSERT_TRACE1("split \"f(o*o\" \"1foo2\"",
+                      EvalError,
+                      hintfmt("invalid regular expression '%s'", "f(o*o"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, concatStringsSep) {
+        ASSERT_TRACE2("concatStringsSep 1 {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument (the separator string) passed to builtins.concatStringsSep"));
+
+        ASSERT_TRACE2("concatStringsSep \"foo\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a set"),
+                      hintfmt("while evaluating the second argument (the list of strings to concat) passed to builtins.concatStringsSep"));
+
+        ASSERT_TRACE2("concatStringsSep \"foo\" [ 1 2 {} ] # TODO: coerce to string is buggy",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "an integer"),
+                      hintfmt("while evaluating one element of the list of strings to concat passed to builtins.concatStringsSep"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, parseDrvName) {
+        ASSERT_TRACE2("parseDrvName 1",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.parseDrvName"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, compareVersions) {
+        ASSERT_TRACE2("compareVersions 1 {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.compareVersions"));
+
+        ASSERT_TRACE2("compareVersions \"abd\" {}",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "a set"),
+                      hintfmt("while evaluating the second argument passed to builtins.compareVersions"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, splitVersion) {
+        ASSERT_TRACE2("splitVersion 1",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the first argument passed to builtins.splitVersion"));
+
+    }
+
+
+    TEST_F(ErrorTraceTest, traceVerbose) {
+    }
+
+
+    /* // Needs different ASSERTs
+    TEST_F(ErrorTraceTest, derivationStrict) {
+        ASSERT_TRACE2("derivationStrict \"\"",
+                      TypeError,
+                      hintfmt("value is %s while a set was expected", "a string"),
+                      hintfmt("while evaluating the argument passed to builtins.derivationStrict"));
+
+        ASSERT_TRACE2("derivationStrict {}",
+                      TypeError,
+                      hintfmt("attribute '%s' missing", "name"),
+                      hintfmt("in the attrset passed as argument to builtins.derivationStrict"));
+
+        ASSERT_TRACE2("derivationStrict { name = 1; }",
+                      TypeError,
+                      hintfmt("value is %s while a string was expected", "an integer"),
+                      hintfmt("while evaluating the `name` attribute passed to builtins.derivationStrict"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; }",
+                      TypeError,
+                      hintfmt("required attribute 'builder' missing"),
+                      hintfmt("while evaluating derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; __structuredAttrs = 15; }",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the `__structuredAttrs` attribute passed to builtins.derivationStrict"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; __ignoreNulls = 15; }",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "an integer"),
+                      hintfmt("while evaluating the `__ignoreNulls` attribute passed to builtins.derivationStrict"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; outputHashMode = 15; }",
+                      TypeError,
+                      hintfmt("invalid value '15' for 'outputHashMode' attribute"),
+                      hintfmt("while evaluating the attribute 'outputHashMode' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; outputHashMode = \"custom\"; }",
+                      TypeError,
+                      hintfmt("invalid value 'custom' for 'outputHashMode' attribute"),
+                      hintfmt("while evaluating the attribute 'outputHashMode' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = {}; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the attribute 'system' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = {}; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"drv\"; }",
+                      TypeError,
+                      hintfmt("invalid derivation output name 'drv'"),
+                      hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = []; }",
+                      TypeError,
+                      hintfmt("derivation cannot have an empty set of outputs"),
+                      hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = [ \"drv\" ]; }",
+                      TypeError,
+                      hintfmt("invalid derivation output name 'drv'"),
+                      hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = [ \"out\" \"out\" ]; }",
+                      TypeError,
+                      hintfmt("duplicate derivation output 'out'"),
+                      hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; __contentAddressed = \"true\"; }",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "a string"),
+                      hintfmt("while evaluating the attribute '__contentAddressed' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; __impure = \"true\"; }",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "a string"),
+                      hintfmt("while evaluating the attribute '__impure' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; __impure = \"true\"; }",
+                      TypeError,
+                      hintfmt("value is %s while a Boolean was expected", "a string"),
+                      hintfmt("while evaluating the attribute '__impure' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = \"foo\"; }",
+                      TypeError,
+                      hintfmt("value is %s while a list was expected", "a string"),
+                      hintfmt("while evaluating the attribute 'args' of derivation 'foo'"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = [ {} ]; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating an element of the argument list"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = [ \"a\" {} ]; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating an element of the argument list"));
+
+        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; FOO = {}; }",
+                      TypeError,
+                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("while evaluating the attribute 'FOO' of derivation 'foo'"));
+
+    }
+    */
+
+} /* namespace nix */
diff --git a/src/libexpr/tests/primops.cc b/src/libexpr/tests/primops.cc
index bcdc7086b..9cdcf64a1 100644
--- a/src/libexpr/tests/primops.cc
+++ b/src/libexpr/tests/primops.cc
@@ -823,4 +823,10 @@ namespace nix {
         for (const auto [n, elem] : enumerate(v.listItems()))
             ASSERT_THAT(*elem, IsStringEq(expected[n]));
     }
+
+    TEST_F(PrimOpTest, genericClosure_not_strict) {
+        // Operator should not be used when startSet is empty
+        auto v = eval("builtins.genericClosure { startSet = []; }");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
 } /* namespace nix */
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index 6d4365652..d4871a8e2 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -404,8 +404,6 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
         return 1;
     } catch (BaseError & e) {
         logError(e.info());
-        if (e.hasTrace() && !loggerSettings.showTrace.get())
-            printError("(use '--show-trace' to show detailed location information)");
         return e.status;
     } catch (std::bad_alloc & e) {
         printError(error + "out of memory");
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 1a1aecea5..e4f0d4677 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -9,9 +9,9 @@ namespace nix {
 
 const std::string nativeSystem = SYSTEM;
 
-void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint)
+void BaseError::addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame)
 {
-    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint });
+    err.traces.push_front(Trace { .pos = std::move(e), .hint = hint, .frame = frame });
 }
 
 // c++ std::exception descendants must have a 'const char* what()' function.
@@ -200,13 +200,125 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
 
-    // traces
-    if (showTrace && !einfo.traces.empty()) {
+    /*
+     * Traces
+     * ------
+     *
+     *  The semantics of traces is a bit weird. We have only one option to
+     *  print them and to make them verbose (--show-trace). In the code they
+     *  are always collected, but they are not printed by default. The code
+     *  also collects more traces when the option is on. This means that there
+     *  is no way to print the simplified traces at all.
+     *
+     *  I (layus) designed the code to attach positions to a restricted set of
+     *  messages. This means that we have  a lot of traces with no position at
+     *  all, including most of the base error messages. For example "type
+     *  error: found a string while a set was expected" has no position, but
+     *  will come with several traces detailing it's precise relation to the
+     *  closest know position. This makes erroring without printing traces
+     *  quite useless.
+     *
+     *  This is why I introduced the idea to always print a few traces on
+     *  error. The number 3 is quite arbitrary, and was selected so as not to
+     *  clutter the console on error. For the same reason, a trace with an
+     *  error position takes more space, and counts as two traces towards the
+     *  limit.
+     *
+     *  The rest is truncated, unless --show-trace is passed. This preserves
+     *  the same bad semantics of --show-trace to both show the trace and
+     *  augment it with new data. Not too sure what is the best course of
+     *  action.
+     *
+     *  The issue is that it is fundamentally hard to provide a trace for a
+     *  lazy language. The trace will only cover the current spine of the
+     *  evaluation, missing things that have been evaluated before. For
+     *  example, most type errors are hard to inspect because there is not
+     *  trace for the faulty value. These errors should really print the faulty
+     *  value itself.
+     *
+     *  In function calls, the --show-trace flag triggers extra traces for each
+     *  function invocation. These work as scopes, allowing to follow the
+     *  current spine of the evaluation graph. Without that flag, the error
+     *  trace should restrict itself to a restricted prefix of that trace,
+     *  until the first scope. If we ever get to such a precise error
+     *  reporting, there would be no need to add an arbitrary limit here. We
+     *  could always print the full trace, and it would just be small without
+     *  the flag.
+     *
+     *  One idea I had is for XxxError.addTrace() to perform nothing if one
+     *  scope has already been traced. Alternatively, we could stop here when
+     *  we encounter such a scope instead of after an arbitrary number of
+     *  traces. This however requires to augment traces with the notion of
+     *  "scope".
+     *
+     *  This is particularly visible in code like evalAttrs(...) where we have
+     *  to make a decision between the two following options.
+     *
+     *  ``` long traces
+     *  inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx)
+     *  {
+     *      try {
+     *          e->eval(*this, env, v);
+     *          if (v.type() != nAttrs)
+     *              throwTypeError("value is %1% while a set was expected", v);
+     *      } catch (Error & e) {
+     *          e.addTrace(pos, errorCtx);
+     *          throw;
+     *      }
+     *  }
+     *  ```
+     *
+     *  ``` short traces
+     *  inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos, std::string_view errorCtx)
+     *  {
+     *      e->eval(*this, env, v);
+     *      try {
+     *          if (v.type() != nAttrs)
+     *              throwTypeError("value is %1% while a set was expected", v);
+     *      } catch (Error & e) {
+     *          e.addTrace(pos, errorCtx);
+     *          throw;
+     *      }
+     *  }
+     *  ```
+     *
+     *  The second example can be rewritten more concisely, but kept in this
+     *  form to highlight the symmetry. The first option adds more information,
+     *  because whatever caused an error down the line, in the generic eval
+     *  function, will get annotated with the code location that uses and
+     *  required it. The second option is less verbose, but does not provide
+     *  any context at all as to where and why a failing value was required.
+     *
+     *  Scopes would fix that, by adding context only when --show-trace is
+     *  passed, and keeping the trace terse otherwise.
+     *
+     */
+
+    // Enough indent to align with with the `... `
+    // prepended to each element of the trace
+    auto ellipsisIndent = "  ";
+
+    bool frameOnly = false;
+    if (!einfo.traces.empty()) {
+        size_t count = 0;
         for (const auto & trace : einfo.traces) {
+            if (!showTrace && count > 3) {
+                oss << "\n" << ANSI_WARNING "(stack trace truncated; use '--show-trace' to show the full trace)" ANSI_NORMAL << "\n";
+                break;
+            }
+
+            if (trace.hint.str().empty()) continue;
+            if (frameOnly && !trace.frame) continue;
+
+            count++;
+            frameOnly = trace.frame;
+
             oss << "\n" << "… " << trace.hint.str() << "\n";
 
             if (trace.pos) {
-                oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
+                count++;
+
+                oss << "\n" << ellipsisIndent << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
 
                 if (auto loc = trace.pos->getCodeLines()) {
                     oss << "\n";
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index c3bb8c0df..7d236028c 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -86,6 +86,7 @@ void printCodeLines(std::ostream & out,
 struct Trace {
     std::shared_ptr<AbstractPos> pos;
     hintformat hint;
+    bool frame;
 };
 
 struct ErrorInfo {
@@ -114,6 +115,8 @@ protected:
 public:
     unsigned int status = 1; // exit status
 
+    BaseError(const BaseError &) = default;
+
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
         : err { .level = lvlError, .msg = hintfmt(args...) }
@@ -152,15 +155,22 @@ public:
     const std::string & msg() const { return calcWhat(); }
     const ErrorInfo & info() const { calcWhat(); return err; }
 
+    void pushTrace(Trace trace)
+    {
+        err.traces.push_front(trace);
+    }
+
     template<typename... Args>
-    void addTrace(std::shared_ptr<AbstractPos> && e, const std::string & fs, const Args & ... args)
+    void addTrace(std::shared_ptr<AbstractPos> && e, std::string_view fs, const Args & ... args)
     {
-        addTrace(std::move(e), hintfmt(fs, args...));
+        addTrace(std::move(e), hintfmt(std::string(fs), args...));
     }
 
-    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint);
+    void addTrace(std::shared_ptr<AbstractPos> && e, hintformat hint, bool frame = false);
 
     bool hasTrace() const { return !err.traces.empty(); }
+
+    const ErrorInfo & info() { return err; };
 };
 
 #define MakeError(newClass, superClass) \
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 4b1202be3..cad7f9c88 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -134,9 +134,9 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     state.forceValue(topLevel, [&]() { return topLevel.determinePos(noPos); });
     PathSet context;
     Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
-    auto topLevelDrv = state.coerceToStorePath(aDrvPath.pos, *aDrvPath.value, context);
+    auto topLevelDrv = state.coerceToStorePath(aDrvPath.pos, *aDrvPath.value, context, "");
     Attr & aOutPath(*topLevel.attrs->find(state.sOutPath));
-    auto topLevelOut = state.coerceToStorePath(aOutPath.pos, *aOutPath.value, context);
+    auto topLevelOut = state.coerceToStorePath(aOutPath.pos, *aOutPath.value, context, "");
 
     /* Realise the resulting store expression. */
     debug("building user environment");
diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index 26db08d80..6ae9460f6 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -97,13 +97,13 @@ struct CmdBundle : InstallableCommand
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
         PathSet context2;
-        auto drvPath = evalState->coerceToStorePath(attr1->pos, *attr1->value, context2);
+        auto drvPath = evalState->coerceToStorePath(attr1->pos, *attr1->value, context2, "");
 
         auto attr2 = vRes->attrs->get(evalState->sOutPath);
         if (!attr2)
             throw Error("the bundler '%s' does not produce a derivation", bundler.what());
 
-        auto outPath = evalState->coerceToStorePath(attr2->pos, *attr2->value, context2);
+        auto outPath = evalState->coerceToStorePath(attr2->pos, *attr2->value, context2, "");
 
         store->buildPaths({
             DerivedPath::Built {
@@ -118,7 +118,7 @@ struct CmdBundle : InstallableCommand
             auto * attr = vRes->attrs->get(evalState->sName);
             if (!attr)
                 throw Error("attribute 'name' missing");
-            outLink = evalState->forceStringNoCtx(*attr->value, attr->pos);
+            outLink = evalState->forceStringNoCtx(*attr->value, attr->pos, "");
         }
 
         // TODO: will crash if not a localFSStore?
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index ba82b5772..ccee074e9 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -112,7 +112,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         else if (raw) {
             stopProgressBar();
-            std::cout << *state->coerceToString(noPos, *v, context);
+            std::cout << *state->coerceToString(noPos, *v, context, "while generating the eval command output");
         }
 
         else if (json) {
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 17ebf12eb..020c1b182 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -126,12 +126,12 @@ static void enumerateOutputs(EvalState & state, Value & vFlake,
     std::function<void(const std::string & name, Value & vProvide, const PosIdx pos)> callback)
 {
     auto pos = vFlake.determinePos(noPos);
-    state.forceAttrs(vFlake, pos);
+    state.forceAttrs(vFlake, pos, "while evaluating a flake to get its outputs");
 
     auto aOutputs = vFlake.attrs->get(state.symbols.create("outputs"));
     assert(aOutputs);
 
-    state.forceAttrs(*aOutputs->value, pos);
+    state.forceAttrs(*aOutputs->value, pos, "while evaluating the outputs of a flake");
 
     auto sHydraJobs = state.symbols.create("hydraJobs");
 
@@ -391,13 +391,13 @@ struct CmdFlakeCheck : FlakeCommand
 
         checkHydraJobs = [&](const std::string & attrPath, Value & v, const PosIdx pos) {
             try {
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (state->isDerivation(v))
                     throw Error("jobset should not be a derivation at top-level");
 
                 for (auto & attr : *v.attrs) {
-                    state->forceAttrs(*attr.value, attr.pos);
+                    state->forceAttrs(*attr.value, attr.pos, "");
                     auto attrPath2 = concatStrings(attrPath, ".", state->symbols[attr.name]);
                     if (state->isDerivation(*attr.value)) {
                         Activity act(*logger, lvlChatty, actUnknown,
@@ -419,7 +419,7 @@ struct CmdFlakeCheck : FlakeCommand
                     fmt("checking NixOS configuration '%s'", attrPath));
                 Bindings & bindings(*state->allocBindings(0));
                 auto vToplevel = findAlongAttrPath(*state, "config.system.build.toplevel", bindings, v).first;
-                state->forceAttrs(*vToplevel, pos);
+                state->forceValue(*vToplevel, pos);
                 if (!state->isDerivation(*vToplevel))
                     throw Error("attribute 'config.system.build.toplevel' is not a derivation");
             } catch (Error & e) {
@@ -433,12 +433,12 @@ struct CmdFlakeCheck : FlakeCommand
                 Activity act(*logger, lvlChatty, actUnknown,
                     fmt("checking template '%s'", attrPath));
 
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (auto attr = v.attrs->get(state->symbols.create("path"))) {
                     if (attr->name == state->symbols.create("path")) {
                         PathSet context;
-                        auto path = state->coerceToPath(attr->pos, *attr->value, context);
+                        auto path = state->coerceToPath(attr->pos, *attr->value, context, "");
                         if (!store->isInStore(path))
                             throw Error("template '%s' has a bad 'path' attribute");
                         // TODO: recursively check the flake in 'path'.
@@ -447,7 +447,7 @@ struct CmdFlakeCheck : FlakeCommand
                     throw Error("template '%s' lacks attribute 'path'", attrPath);
 
                 if (auto attr = v.attrs->get(state->symbols.create("description")))
-                    state->forceStringNoCtx(*attr->value, attr->pos);
+                    state->forceStringNoCtx(*attr->value, attr->pos, "");
                 else
                     throw Error("template '%s' lacks attribute 'description'", attrPath);
 
@@ -504,11 +504,11 @@ struct CmdFlakeCheck : FlakeCommand
                             warn("flake output attribute '%s' is deprecated; use '%s' instead", name, replacement);
 
                         if (name == "checks") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     auto drvPath = checkDerivation(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -524,7 +524,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "formatter") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -535,11 +535,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "packages" || name == "devShells") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkDerivation(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -548,11 +548,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "apps") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkApp(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
@@ -561,7 +561,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultPackage" || name == "devShell") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -572,7 +572,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultApp") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -583,7 +583,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "legacyPackages") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(state->symbols[attr.name], attr.pos);
                                 // FIXME: do getDerivations?
@@ -594,7 +594,7 @@ struct CmdFlakeCheck : FlakeCommand
                             checkOverlay(name, vOutput, pos);
 
                         else if (name == "overlays") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkOverlay(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
@@ -604,14 +604,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkModule(name, vOutput, pos);
 
                         else if (name == "nixosModules") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkModule(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
                         }
 
                         else if (name == "nixosConfigurations") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkNixOSConfiguration(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
@@ -624,14 +624,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkTemplate(name, vOutput, pos);
 
                         else if (name == "templates") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkTemplate(fmt("%s.%s", name, state->symbols[attr.name]),
                                     *attr.value, attr.pos);
                         }
 
                         else if (name == "defaultBundler") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
@@ -642,11 +642,11 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "bundlers") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 const auto & attr_name = state->symbols[attr.name];
                                 checkSystemName(attr_name, attr.pos);
-                                state->forceAttrs(*attr.value, attr.pos);
+                                state->forceAttrs(*attr.value, attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     checkBundler(
                                         fmt("%s.%s.%s", name, attr_name, state->symbols[attr2.name]),
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 2c6309c81..d3d2f5b16 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -199,7 +199,7 @@ static void showHelp(std::vector<std::string> subcommand, MultiCommand & topleve
     if (!attr)
         throw UsageError("Nix has no subcommand '%s'", concatStringsSep("", subcommand));
 
-    auto markdown = state.forceString(*attr->value);
+    auto markdown = state.forceString(*attr->value, noPos, "while evaluating the lowdown help text");
 
     RunPager pager;
     std::cout << renderMarkdownToTerminal(markdown) << "\n";
diff --git a/src/nix/prefetch.cc b/src/nix/prefetch.cc
index ce3288dc1..fc3823406 100644
--- a/src/nix/prefetch.cc
+++ b/src/nix/prefetch.cc
@@ -28,17 +28,17 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
     Value vMirrors;
     // FIXME: use nixpkgs flake
     state.eval(state.parseExprFromString("import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>", "."), vMirrors);
-    state.forceAttrs(vMirrors, noPos);
+    state.forceAttrs(vMirrors, noPos, "while evaluating the set of all mirrors");
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
     if (mirrorList == vMirrors.attrs->end())
         throw Error("unknown mirror name '%s'", mirrorName);
-    state.forceList(*mirrorList->value, noPos);
+    state.forceList(*mirrorList->value, noPos, "while evaluating one mirror configuration");
 
     if (mirrorList->value->listSize() < 1)
         throw Error("mirror URL '%s' did not expand to anything", url);
 
-    std::string mirror(state.forceString(*mirrorList->value->listElems()[0]));
+    std::string mirror(state.forceString(*mirrorList->value->listElems()[0], noPos, "while evaluating the first available mirror"));
     return mirror + (hasSuffix(mirror, "/") ? "" : "/") + s.substr(p + 1);
 }
 
@@ -196,29 +196,29 @@ static int main_nix_prefetch_url(int argc, char * * argv)
             Value vRoot;
             state->evalFile(path, vRoot);
             Value & v(*findAlongAttrPath(*state, attrPath, autoArgs, vRoot).first);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "while evaluating the source attribute to prefetch");
 
             /* Extract the URL. */
             auto * attr = v.attrs->get(state->symbols.create("urls"));
             if (!attr)
                 throw Error("attribute 'urls' missing");
-            state->forceList(*attr->value, noPos);
+            state->forceList(*attr->value, noPos, "while evaluating the urls to prefetch");
             if (attr->value->listSize() < 1)
                 throw Error("'urls' list is empty");
-            url = state->forceString(*attr->value->listElems()[0]);
+            url = state->forceString(*attr->value->listElems()[0], noPos, "while evaluating the first url from the urls list");
 
             /* Extract the hash mode. */
             auto attr2 = v.attrs->get(state->symbols.create("outputHashMode"));
             if (!attr2)
                 printInfo("warning: this does not look like a fetchurl call");
             else
-                unpack = state->forceString(*attr2->value) == "recursive";
+                unpack = state->forceString(*attr2->value, noPos, "while evaluating the outputHashMode of the source to prefetch") == "recursive";
 
             /* Extract the name. */
             if (!name) {
                 auto attr3 = v.attrs->get(state->symbols.create("name"));
                 if (!attr3)
-                    name = state->forceString(*attr3->value);
+                    name = state->forceString(*attr3->value, noPos, "while evaluating the name of the source to prefetch");
             }
         }
 
diff --git a/src/nix/upgrade-nix.cc b/src/nix/upgrade-nix.cc
index 2d2453395..17796d6b8 100644
--- a/src/nix/upgrade-nix.cc
+++ b/src/nix/upgrade-nix.cc
@@ -144,7 +144,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         Bindings & bindings(*state->allocBindings(0));
         auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v).first;
 
-        return store->parseStorePath(state->forceString(*v2));
+        return store->parseStorePath(state->forceString(*v2, noPos, "while evaluating the path tho latest nix version"));
     }
 };