Revert "Revert "Merge pull request #6204 from layus/coerce-string""

This reverts commit 9b33ef3879a764bed4cc2404a08344c3a697a646.

1 files changed, 243 insertions, 323 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 9bc20a502..277cbb5f9 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -318,7 +318,7 @@ 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 +414,44 @@ 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 +684,7 @@ 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 +862,14 @@ 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 +946,7 @@ 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 +956,7 @@ 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 +1106,7 @@ 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 +1124,31 @@ void EvalState::eval(Expr * e, Value & v)
 }
 
 
-inline bool EvalState::evalBool(Env & env, Expr * e)
-{
-    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)
+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(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 +1221,7 @@ 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 +1249,11 @@ 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 +1350,14 @@ 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 +1452,12 @@ 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 +1469,15 @@ 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 +1495,15 @@ 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 +1526,15 @@ 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 +1553,17 @@ 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 +1598,20 @@ 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 +1624,18 @@ 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 +1699,12 @@ 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 +1727,17 @@ 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 +1745,7 @@ 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 +1753,7 @@ 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 +1761,33 @@ 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 +1826,18 @@ 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 +1914,20 @@ 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, false, firstType == nString, !first, "while evaluating a path segment");
             sSize += part->size();
             s.emplace_back(std::move(part));
         }
@@ -2041,7 +1941,7 @@ 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 +1991,47 @@ 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 +2041,30 @@ 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 +2087,19 @@ 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 +2123,15 @@ 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, coerceMore, copyToStore,
+                "while evaluating the result of the `toString` attribute").toOwned();
     }
 
     return {};
 }
 
 BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet & context,
-    bool coerceMore, bool copyToStore, bool canonicalizePath)
+    bool coerceMore, bool copyToStore, bool canonicalizePath, std::string_view errorCtx)
 {
     forceValue(v, pos);
 
@@ -2236,12 +2155,12 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
             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);
+            error("cannot coerce a set to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+        return coerceToString(pos, *i->value, context, coerceMore, copyToStore, canonicalizePath, errorCtx);
     }
 
     if (v.type() == nExternal)
-        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore);
+        return v.external->coerceToString(positions[pos], context, coerceMore, copyToStore, errorCtx);
 
     if (coerceMore) {
         /* Note that `false' is represented as an empty string for
@@ -2255,7 +2174,13 @@ 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, coerceMore, copyToStore, canonicalizePath,
+                            "while evaluating one element of the list");
+                } catch (Error & e) {
+                    e.addTrace(positions[pos], errorCtx);
+                    throw;
+                }
                 if (n < v.listSize() - 1
                     /* !!! not quite correct */
                     && (!v2->isList() || v2->listSize() != 0))
@@ -2265,14 +2190,14 @@ 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 +2218,25 @@ 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, false, false, true, errorCtx).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, false, false, true, errorCtx).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 +2256,6 @@ 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 +2274,7 @@ 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 +2284,7 @@ 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 +2292,7 @@ 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 +2309,7 @@ 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>();
     }
 }
 
@@ -2514,12 +2433,13 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const
 {
-    throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType()),
-        .errPos = pos
+    auto e = TypeError({
+        .msg = hintfmt("cannot coerce %1% to a string", showType())
     });
+    e.addTrace(pos, errorCtx);
+    throw e;
 }