Add tests for error traces, and fixes

8 files changed, 1377 insertions, 140 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 277cbb5f9..16b1c2f23 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>
@@ -1927,7 +1929,9 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
             /* 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, "while evaluating a path segment");
+            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));
         }
@@ -2123,15 +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,
-                "while evaluating the result of the `toString` attribute").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, std::string_view errorCtx)
+BackedStringView EvalState::coerceToString(const PosIdx pos, Value &v, PathSet &context,
+    std::string_view errorCtx, bool coerceMore, bool copyToStore, bool canonicalizePath)
 {
     forceValue(v, pos);
 
@@ -2154,13 +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())
-            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 (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, errorCtx);
+    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
@@ -2175,8 +2190,9 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
             std::string result;
             for (auto [n, v2] : enumerate(v.listItems())) {
                 try {
-                    result += *coerceToString(noPos, *v2, context, coerceMore, copyToStore, canonicalizePath,
-                            "while evaluating one element of the list");
+                    result += *coerceToString(noPos, *v2, context,
+                            "while evaluating one element of the list",
+                            coerceMore, copyToStore, canonicalizePath);
                 } catch (Error & e) {
                     e.addTrace(positions[pos], errorCtx);
                     throw;
@@ -2190,7 +2206,9 @@ BackedStringView EvalState::coerceToString(const PosIdx pos, Value & v, PathSet
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    error("cannot coerce %1% to a string", showType(v))
+        .withTrace(pos, errorCtx)
+        .debugThrow<TypeError>();
 }
 
 
@@ -2220,7 +2238,7 @@ StorePath EvalState::copyPathToStore(PathSet & context, const Path & path)
 
 Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (path == "" || path[0] != '/')
         error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
     return path;
@@ -2229,7 +2247,7 @@ Path EvalState::coerceToPath(const PosIdx pos, Value & v, PathSet & context, std
 
 StorePath EvalState::coerceToStorePath(const PosIdx pos, Value & v, PathSet & context, std::string_view errorCtx)
 {
-    auto path = coerceToString(pos, v, context, false, false, true, errorCtx).toOwned();
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
     if (auto storePath = store->maybeParseStorePath(path))
         return *storePath;
     error("path '%1%' is not in the Nix store", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
@@ -2433,13 +2451,11 @@ void EvalState::printStats()
 }
 
 
-std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const
+std::string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    auto e = TypeError({
+    throw TypeError({
         .msg = hintfmt("cannot coerce %1% to a string", showType())
     });
-    e.addTrace(pos, errorCtx);
-    throw e;
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 46b8cbaa5..1d2e5005b 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -375,9 +375,9 @@ 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,
-        std::string_view errorCtx = "");
+        bool canonicalizePath = true);
 
     StorePath copyPathToStore(PathSet & context, const Path & path);
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index fc4be5678..336eb274d 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -264,7 +264,7 @@ static Flake getFlake(
                 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(
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 9cff4b365..2b340fe52 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -350,26 +350,22 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     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,
-           "while evaluating the first element of the argument passed to builtins.exec").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,
-                    "while evaluating an element of the argument passed to builtins.exec").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);
@@ -598,7 +594,8 @@ struct CompareValues
                     state.error("cannot compare %s with %s; values of that type are incomparable", showType(*v1), showType(*v2)).debugThrow<EvalError>();
             }
         } catch (Error & e) {
-            e.addTrace(nullptr, errorCtx);
+            if (!errorCtx.empty())
+                e.addTrace(nullptr, errorCtx);
             throw;
         }
     }
@@ -620,15 +617,7 @@ static Bindings::iterator getAttr(
 {
     Bindings::iterator value = attrSet->find(attrSym);
     if (value == attrSet->end()) {
-        throw TypeError({
-            .msg = hintfmt("attribute '%s' missing %s", state.symbols[attrSym], normaltxt(errorCtx)),
-            .errPos = state.positions[attrSet->pos],
-        });
-        // TODO XXX
-        // 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;
 }
@@ -801,8 +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,
-                "while evaluating the error message passed to builtins.addErrorContext").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);
         throw;
     }
 }
@@ -1006,6 +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
@@ -1016,32 +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, "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, state.sName, args[0]->attrs, "in the attrset passed as argument to builtins.derivationStrict");
+    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, "while evaluating the `name` attribute passed to builtins.derivationStrict");
+        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, "while evaluating the `__structuredAttrs` attribute passed to builtins.derivationStrict"))
+    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, "while evaluating the `__ignoreNulls` attribute passed to builtins.derivationStrict");
+    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;
@@ -1058,7 +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);
@@ -1069,7 +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]
                 }));
         };
 
@@ -1079,7 +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. */
@@ -1089,34 +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,
-                        "while evaluating the `__contentAddressed` attribute passed to builtins.derivationStrict");
+                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,
-                        "while evaluating the 'impure' attribute passed to builtins.derivationStrict");
+                isImpure = state.forceBool(*i->value, noPos, context_below);
                 if (isImpure)
                     settings.requireExperimentalFeature(Xp::ImpureDerivations);
             }
@@ -1124,11 +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,
-                        "while evaluating the `args` attribute passed to builtins.derivationStrict");
+                state.forceList(*i->value, noPos, context_below);
                 for (auto elem : i->value->listItems()) {
-                    auto s = state.coerceToString(posDrvName, *elem, context, true,
-                            "while evaluating an element of the `args` argument passed to builtins.derivationStrict").toOwned();
+                    auto s = state.coerceToString(noPos, *elem, context,
+                            "while evaluating an element of the argument list",
+                            true).toOwned();
                     drv.args.push_back(s);
                 }
             }
@@ -1141,29 +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, "while evaluating the `builder` attribute passed to builtins.derivationStrict");
+                        drv.builder = state.forceString(*i->value, context, noPos, context_below);
                     else if (i->name == state.sSystem)
-                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `system` attribute passed to builtins.derivationStrict");
+                        drv.platform = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHash)
-                        outputHash = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHash` attribute passed to builtins.derivationStrict");
+                        outputHash = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashAlgo)
-                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHashAlgo` attribute passed to builtins.derivationStrict");
+                        outputHashAlgo = state.forceStringNoCtx(*i->value, noPos, context_below);
                     else if (i->name == state.sOutputHashMode)
-                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName, "while evaluating the `outputHashMode` attribute passed to builtins.derivationStrict"));
+                        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, "while evaluating the `outputs` attribute passed to builtins.derivationStrict");
+                        state.forceList(*i->value, noPos, context_below);
                         Strings ss;
                         for (auto elem : i->value->listItems())
-                            ss.emplace_back(state.forceStringNoCtx(*elem, posDrvName, "while evaluating an element of the `outputs` attribute passed to builtins.derivationStrict"));
+                            ss.emplace_back(state.forceStringNoCtx(*elem, noPos, context_below));
                         handleOutputs(ss);
                     }
 
                 } else {
-                    auto s = state.coerceToString(i->pos, *i->value, context, true, "while evaluating an attribute passed to builtins.derivationStrict").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);
@@ -1177,8 +1206,8 @@ static void prim_derivationStrict(EvalState & state, const PosIdx pos, Value * *
             }
 
         } catch (Error & e) {
-            e.addTrace(nullptr,
-                hintfmt("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;
         }
@@ -1223,20 +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) {
@@ -1247,7 +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));
@@ -1268,7 +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);
@@ -1312,7 +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);
@@ -1345,11 +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 {
@@ -1492,7 +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, "while evaluating the first argument passed to builtins.baseNameOf")), 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({
@@ -1512,7 +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, "while evaluating the first argument passed to builtins.dirOf");
+    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);
 }
@@ -1578,8 +1612,9 @@ static void prim_findFile(EvalState & state, const PosIdx pos, Value * * args, V
         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,
-                "while evaluating the `path` attribute of an element of the list passed to builtins.findFile").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);
@@ -2622,14 +2657,9 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
 
     for (unsigned int n = 0; n < listSize; ++n) {
         Value * vElem = listElems[n];
-        try {
-            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++;
-        } 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());
@@ -3013,13 +3043,13 @@ static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Va
     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);
@@ -3067,6 +3097,8 @@ 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, noPos, "while evaluating the ordering function passed to builtins.sort")(a, b);
 
@@ -3227,12 +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)), "while evaluating the return value of the function passed to buitlins.concatMap");
-        } 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();
     }
 
@@ -3412,7 +3439,7 @@ static void prim_lessThan(EvalState & state, const PosIdx pos, Value * * args, V
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     // pos is exact here, no need for a message.
-    CompareValues comp(state, pos, "");
+    CompareValues comp(state, noPos, "");
     v.mkBool(comp(args[0], args[1]));
 }
 
@@ -3439,7 +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, "while evaluating the first argument passed to builtins.toString");
+    auto s = state.coerceToString(pos, *args[0], context,
+            "while evaluating the first argument passed to builtins.toString",
+            true, false);
     v.mkString(*s, context);
 }
 
@@ -3791,21 +3820,18 @@ static void prim_replaceStrings(EvalState & state, const PosIdx pos, Value * * a
     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, "while evaluating one of the strings to replace in builtins.replaceStrings"));
+        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, "while evaluating one of the replacement strings of builtins.replaceStrings");
+        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));
     }
 
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index c9c93bdba..c41bd60b6 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -22,7 +22,9 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
         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, "while evaluating the `url` attribute passed to builtins.fetchMercurial").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.
@@ -48,7 +50,9 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
             });
 
     } else
-        url = state.coerceToString(pos, *args[0], context, false, false, "while evaluating the first argument passed to builtins.fetchMercurial").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 cc318cbaa..83d93b75c 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -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, "while evaluating the first argument passed to the fetcher").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;
diff --git a/src/libexpr/tests/error_traces.cc b/src/libexpr/tests/error_traces.cc
index 8741ecdd2..7d6bd2111 100644
--- a/src/libexpr/tests/error_traces.cc
+++ b/src/libexpr/tests/error_traces.cc
@@ -10,16 +10,56 @@ namespace nix {
     // 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
+        );
+    }
+
+
 #define ASSERT_TRACE1(args, type, message)                                  \
         ASSERT_THROW(                                                       \
+            std::string expr(args);                                         \
+            std::string name = expr.substr(0, expr.find(" "));              \
             try {                                                           \
-                eval("builtins." args);                                     \
+                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", "genericClosure"))); \
+                          PrintToString(hintfmt("while calling the '%s' builtin", name))); \
                 throw;                                                      \
             }                                                               \
             , type                                                          \
@@ -27,39 +67,42 @@ namespace nix {
 
 #define ASSERT_TRACE2(args, type, message, context)                         \
         ASSERT_THROW(                                                       \
+            std::string expr(args);                                         \
+            std::string name = expr.substr(0, expr.find(" "));              \
             try {                                                           \
-                eval("builtins." args);                                     \
+                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", "genericClosure"))); \
+                          PrintToString(hintfmt("while calling the '%s' builtin", name))); \
                 throw;                                                      \
             }                                                               \
             , type                                                          \
         )
 
-    TEST_F(ErrorTraceTest, genericClosure) { \
+    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_TRACE1("genericClosure {}",
+        ASSERT_TRACE2("genericClosure {}",
                       TypeError,
-                      hintfmt("attribute '%s' missing %s", "startSet", normaltxt("in the attrset passed as argument to builtins.genericClosure")));
+                      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"));
 
-        // Okay:      "genericClosure { startSet = []; }"
-
         ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = true; }",
                       TypeError,
                       hintfmt("value is %s while a function was expected", "a Boolean"),
@@ -68,16 +111,17 @@ namespace nix {
         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")); // TODO: inconsistent naming
+                      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_TRACE1("genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
+        ASSERT_TRACE2("genericClosure { startSet = [{ key = 1;}]; operator = item: [ {} ]; }",
                       TypeError,
-                      hintfmt("attribute '%s' missing %s", "key", normaltxt("in one of the attrsets generated by (or initially passed to) builtins.genericClosure")));
+                      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,
@@ -91,4 +135,1149 @@ namespace nix {
 
     }
 
+
+    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/value.hh b/src/libexpr/value.hh
index 7d3f6d700..508dbe218 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -89,7 +89,7 @@ class ExternalValueBase
     /* Coerce the value to a string. Defaults to uncoercable, i.e. throws an
      * error.
      */
-    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore, std::string_view errorCtx) const;
+    virtual std::string coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const;
 
     /* Compare to another value of the same type. Defaults to uncomparable,
      * i.e. always false.