add pos to errorinfo, remove from hints

src/error-demo/error-demo.cc

@@ -126,17 +126,28 @@ int main()
     // Error with previous and next lines of code.
     logError(
         ErrorInfo { .name = "error name",
-                    .description = "error description",
+                    .description = "error with code lines",
                     .hint = hintfmt("this hint has %1% templated %2%!!",
                         "yellow",
                         "values"),
                     .nixCode = NixCode {
                         .errPos = Pos(problem_file, 40, 13),
-                        .prevLineOfCode = std::optional("previous line of code"),
+                        .prevLineOfCode = "previous line of code",
                         .errLineOfCode = "this is the problem line of code",
-                        .nextLineOfCode = std::optional("next line of code"),
+                        .nextLineOfCode = "next line of code",
                     }});
 
 
+    // Error without lines of code.
+    logError(
+        ErrorInfo { .name = "error name",
+                    .description = "error without any code lines.",
+                    .hint = hintfmt("this hint has %1% templated %2%!!",
+                        "yellow",
+                        "values"),
+                    .nixCode = NixCode {
+                        .errPos = Pos(problem_file, 40, 13)
+                    }});
+
     return 0;
 }

src/libexpr/attr-set.hh

@@ -76,7 +76,12 @@ public:
     {
         auto a = get(name);
         if (!a)
-            throw Error("attribute '%s' missing, at %s", name, pos);
+            throw Error(
+                ErrorInfo { 
+                    .hint = hintfmt("attribute '%s' missing", name),
+                    .nixCode = NixCode { .errPos = pos }
+                });
+
         return *a;
     }
 

src/libexpr/eval-inline.hh

@@ -9,7 +9,11 @@ namespace nix {
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const Pos & pos))
 {
-    throw EvalError(s, pos);
+    throw EvalError(
+        ErrorInfo { 
+            .hint = hintfmt(s),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
@@ -20,7 +24,11 @@ LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v, const Pos & pos))
 {
-    throw TypeError(s, showType(v), pos);
+    throw TypeError(
+        ErrorInfo { 
+            .hint = hintfmt(s, showType(v)),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 
@@ -43,7 +51,7 @@ void EvalState::forceValue(Value & v, const Pos & pos)
     else if (v.type == tApp)
         callFunction(*v.app.left, *v.app.right, v, noPos);
     else if (v.type == tBlackhole)
-        throwEvalError("infinite recursion encountered, at %1%", pos);
+        throwEvalError("infinite recursion encountered", pos);
 }
 
 
@@ -59,7 +67,7 @@ inline void EvalState::forceAttrs(Value & v, const Pos & pos)
 {
     forceValue(v);
     if (v.type != tAttrs)
-        throwTypeError("value is %1% while a set was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a set was expected", v, pos);
 }
 
 
@@ -75,7 +83,7 @@ inline void EvalState::forceList(Value & v, const Pos & pos)
 {
     forceValue(v);
     if (!v.isList())
-        throwTypeError("value is %1% while a list was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a list was expected", v, pos);
 }
 
 /* Note: Various places expect the allocated memory to be zeroed. */

src/libexpr/eval.cc

@@ -498,7 +498,11 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const Pos & pos))
 {
-    throw EvalError(s, s2, pos);
+    throw EvalError(
+        ErrorInfo { 
+            .hint = hintfmt(s, s2),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3))
@@ -508,7 +512,11 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, con
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3, const Pos & pos))
 {
-    throw EvalError(s, s2, s3, pos);
+    throw EvalError(
+        ErrorInfo { 
+            .hint = hintfmt(s, s2, s3),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const Symbol & sym, const Pos & p1, const Pos & p2))
@@ -518,7 +526,11 @@ LocalNoInlineNoReturn(void throwEvalError(const char * s, const Symbol & sym, co
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Pos & pos))
 {
-    throw TypeError(s, pos);
+    throw TypeError(
+        ErrorInfo { 
+            .hint = hintfmt(s),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
@@ -528,17 +540,29 @@ LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const ExprLambda & fun, const Symbol & s2, const Pos & pos))
 {
-    throw TypeError(s, fun.showNamePos(), s2, pos);
+    throw TypeError(
+        ErrorInfo { 
+            .hint = hintfmt(s, fun.showNamePos(), s2),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwAssertionError(const char * s, const string & s1, const Pos & pos))
 {
-    throw AssertionError(s, s1, pos);
+    throw AssertionError(
+        ErrorInfo { 
+            .hint = hintfmt(s, s1),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwUndefinedVarError(const char * s, const string & s1, const Pos & pos))
 {
-    throw UndefinedVarError(s, s1, pos);
+    throw UndefinedVarError(
+        ErrorInfo { 
+            .hint = hintfmt(s, s1),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2))
@@ -804,7 +828,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const Pos & pos)
     Value v;
     e->eval(*this, env, v);
     if (v.type != tBool)
-        throwTypeError("value is %1% while a Boolean was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a Boolean was expected", v, pos);
     return v.boolean;
 }
 
@@ -1006,7 +1030,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
             } else {
                 state.forceAttrs(*vAttrs, pos);
                 if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
-                    throwEvalError("attribute '%1%' missing, at %2%", name, pos);
+                    throwEvalError("attribute '%1%' missing", name, pos);
             }
             vAttrs = j->value;
             pos2 = j->pos;
@@ -1132,7 +1156,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
     }
 
     if (fun.type != tLambda)
-        throwTypeError("attempt to call something which is not a function but %1%, at %2%", fun, pos);
+        throwTypeError("attempt to call something which is not a function but %1%", fun, pos);
 
     ExprLambda & lambda(*fun.lambda.fun);
 
@@ -1160,7 +1184,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
         for (auto & i : lambda.formals->formals) {
             Bindings::iterator j = arg.attrs->find(i.name);
             if (j == arg.attrs->end()) {
-                if (!i.def) throwTypeError("%1% called without required argument '%2%', at %3%",
+                if (!i.def) throwTypeError("%1% called without required argument '%2%'",
                     lambda, i.name, pos);
                 env2.values[displ++] = i.def->maybeThunk(*this, env2);
             } else {
@@ -1176,7 +1200,7 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
                user. */
             for (auto & i : *arg.attrs)
                 if (lambda.formals->argNames.find(i.name) == lambda.formals->argNames.end())
-                    throwTypeError("%1% called with unexpected argument '%2%', at %3%", lambda, i.name, pos);
+                    throwTypeError("%1% called with unexpected argument '%2%'", lambda, i.name, pos);
             abort(); // can't happen
         }
     }
@@ -1417,14 +1441,14 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
                 nf = n;
                 nf += vTmp.fpoint;
             } else
-                throwEvalError("cannot add %1% to an integer, at %2%", showType(vTmp), pos);
+                throwEvalError("cannot add %1% to an integer", showType(vTmp), pos);
         } else if (firstType == tFloat) {
             if (vTmp.type == tInt) {
                 nf += vTmp.integer;
             } else if (vTmp.type == tFloat) {
                 nf += vTmp.fpoint;
             } else
-                throwEvalError("cannot add %1% to a float, at %2%", showType(vTmp), pos);
+                throwEvalError("cannot add %1% to a float", showType(vTmp), pos);
         } else
             s << state.coerceToString(pos, vTmp, context, false, firstType == tString);
     }
@@ -1435,7 +1459,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
         mkFloat(v, nf);
     else if (firstType == tPath) {
         if (!context.empty())
-            throwEvalError("a string that refers to a store path cannot be appended to a path, at %1%", pos);
+            throwEvalError("a string that refers to a store path cannot be appended to a path", pos);
         auto path = canonPath(s.str());
         mkPath(v, path.c_str());
     } else
@@ -1484,7 +1508,7 @@ NixInt EvalState::forceInt(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
     if (v.type != tInt)
-        throwTypeError("value is %1% while an integer was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while an integer was expected", v, pos);
     return v.integer;
 }
 
@@ -1495,7 +1519,7 @@ NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
     if (v.type == tInt)
         return v.integer;
     else if (v.type != tFloat)
-        throwTypeError("value is %1% while a float was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a float was expected", v, pos);
     return v.fpoint;
 }
 
@@ -1504,7 +1528,7 @@ bool EvalState::forceBool(Value & v, const Pos & pos)
 {
     forceValue(v);
     if (v.type != tBool)
-        throwTypeError("value is %1% while a Boolean was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a Boolean was expected", v, pos);
     return v.boolean;
 }
 
@@ -1519,7 +1543,7 @@ void EvalState::forceFunction(Value & v, const Pos & pos)
 {
     forceValue(v);
     if (v.type != tLambda && v.type != tPrimOp && v.type != tPrimOpApp && !isFunctor(v))
-        throwTypeError("value is %1% while a function was expected, at %2%", v, pos);
+        throwTypeError("value is %1% while a function was expected", v, pos);
 }
 
 
@@ -1528,7 +1552,7 @@ string EvalState::forceString(Value & v, const Pos & pos)
     forceValue(v, pos);
     if (v.type != tString) {
         if (pos)
-            throwTypeError("value is %1% while a string was expected, at %2%", v, pos);
+            throwTypeError("value is %1% while a string was expected", v, pos);
         else
             throwTypeError("value is %1% while a string was expected", v);
     }
@@ -1557,7 +1581,7 @@ string EvalState::forceStringNoCtx(Value & v, const Pos & pos)
     string s = forceString(v, pos);
     if (v.string.context) {
         if (pos)
-            throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%'), at %3%",
+            throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
                 v.string.s, v.string.context[0], pos);
         else
             throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
@@ -1614,7 +1638,7 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
             return *maybeString;
         }
         auto i = v.attrs->find(sOutPath);
-        if (i == v.attrs->end()) throwTypeError("cannot coerce a set to a string, at %1%", pos);
+        if (i == v.attrs->end()) throwTypeError("cannot coerce a set to a string", pos);
         return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
     }
 
@@ -1645,7 +1669,7 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
         }
     }
 
-    throwTypeError("cannot coerce %1% to a string, at %2%", v, pos);
+    throwTypeError("cannot coerce %1% to a string", v, pos);
 }
 
 
@@ -1676,7 +1700,7 @@ Path EvalState::coerceToPath(const Pos & pos, Value & v, PathSet & context)
 {
     string path = coerceToString(pos, v, context, false, false);
     if (path == "" || path[0] != '/')
-        throwEvalError("string '%1%' doesn't represent an absolute path, at %2%", path, pos);
+        throwEvalError("string '%1%' doesn't represent an absolute path", path, pos);
     return path;
 }
 
@@ -1883,7 +1907,11 @@ void EvalState::printStats()
 
 string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    throw TypeError("cannot coerce %1% to a string, at %2%", showType(), pos);
+    throw TypeError(
+        ErrorInfo { 
+            .hint = hintfmt("cannot coerce %1% to a string", showType()),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 

src/libexpr/parser.y

@@ -404,7 +404,12 @@ expr_simple
   | URI {
       static bool noURLLiterals = settings.isExperimentalFeatureEnabled("no-url-literals");
       if (noURLLiterals)
-          throw ParseError("URL literals are disabled, at %s", CUR_POS);
+          throw ParseError(
+              ErrorInfo { 
+                  .hint = hintfmt("URL literals are disabled"),
+                  .nixCode = NixCode { .errPos = CUR_POS }
+              });
+
       $$ = new ExprString(data->symbols.create($1));
   }
   | '(' expr ')' { $$ = $2; }
@@ -669,10 +674,12 @@ Path EvalState::findFile(SearchPath & searchPath, const string & path, const Pos
         Path res = r.second + suffix;
         if (pathExists(res)) return canonPath(res);
     }
-    string f = 
+
-        "file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)"
+    throw ThrownError(
-        + string(pos ? ", at %2%" : "");
+        ErrorInfo { 
-    throw ThrownError(f, path, pos);
+            .hint = hintfmt("file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)", path),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 

src/libexpr/primops.cc

@@ -94,8 +94,13 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError("cannot import '%1%', since path '%2%' is not valid, at %3%",
+        throw EvalError(
-                        path, e.path, pos);
+            ErrorInfo { 
+                .hint = hintfmt("cannot import '%1%', since path '%2%' is not valid",
+                        path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
+
     }
 
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));
@@ -171,8 +176,13 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError("cannot import '%1%', since path '%2%' is not valid, at %3%"
+        throw EvalError(
-           , path, e.path, pos);
+            ErrorInfo {
+                .hint = hintfmt(
+                    "cannot import '%1%', since path '%2%' is not valid",
+                   path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 
     path = state.checkSourcePath(path);
@@ -207,7 +217,11 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0) {
-        throw EvalError("at least one argument to 'exec' required, at %1%", pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("at least one argument to 'exec' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
     PathSet context;
     auto program = state.coerceToString(pos, *elems[0], context, false, false);
@@ -218,9 +232,12 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError("cannot execute '%1%', since path '%2%' is not valid, at %3%"
+        throw EvalError(
-           , program, e.path, pos);
+            ErrorInfo {
-    }
+                .hint = hintfmt("cannot execute '%1%', since path '%2%' is not valid"
+                     , program, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });}
 
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
@@ -371,7 +388,11 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator startSet =
         args[0]->attrs->find(state.symbols.create("startSet"));
     if (startSet == args[0]->attrs->end())
-        throw EvalError("attribute 'startSet' required, at %1%", pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("attribute 'startSet' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     state.forceList(*startSet->value, pos);
 
     ValueList workSet;
@@ -382,7 +403,11 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator op =
         args[0]->attrs->find(state.symbols.create("operator"));
     if (op == args[0]->attrs->end())
-        throw EvalError("attribute 'operator' required, at %1%", pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("attribute 'operator' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     state.forceValue(*op->value);
 
     /* Construct the closure by applying the operator to element of
@@ -401,7 +426,11 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         Bindings::iterator key =
             e->attrs->find(state.symbols.create("key"));
         if (key == e->attrs->end())
-            throw EvalError("attribute 'key' required, at %1%", pos);
+            throw EvalError(
+                ErrorInfo {
+                    .hint = hintfmt("attribute 'key' required"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         state.forceValue(*key->value);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -533,7 +562,11 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     /* Figure out the name first (for stack backtraces). */
     Bindings::iterator attr = args[0]->attrs->find(state.sName);
     if (attr == args[0]->attrs->end())
-        throw EvalError("required attribute 'name' missing, at %1%", pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("required attribute 'name' missing"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     string drvName;
     Pos & posDrvName(*attr->pos);
     try {
@@ -576,25 +609,45 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         auto handleHashMode = [&](const std::string & s) {
             if (s == "recursive") outputHashRecursive = true;
             else if (s == "flat") outputHashRecursive = false;
-            else throw EvalError("invalid value '%s' for 'outputHashMode' attribute, at %s", s, posDrvName);
+            else 
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("invalid value '%s' for 'outputHashMode' attribute", s),
+                        .nixCode = NixCode { .errPos = posDrvName }
+                    });
         };
 
         auto handleOutputs = [&](const Strings & ss) {
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    throw EvalError("duplicate derivation output '%1%', at %2%", j, posDrvName);
+                    throw EvalError(
+                        ErrorInfo { 
+                            .hint = hintfmt("duplicate derivation output '%1%'", j),
+                            .nixCode = NixCode { .errPos = posDrvName }
+                        });
+
+
                 /* !!! Check whether j is a valid attribute
                    name. */
                 /* Derivations cannot be named ‘drv’, because
                    then we'd have an attribute ‘drvPath’ in
                    the resulting set. */
                 if (j == "drv")
-                    throw EvalError("invalid derivation output name 'drv', at %1%", posDrvName);
+                    throw EvalError(
+                        ErrorInfo { 
+                            .hint = hintfmt("invalid derivation output name 'drv'" ),
+                            .nixCode = NixCode { .errPos = posDrvName }
+                        });
+
                 outputs.insert(j);
             }
             if (outputs.empty())
-                throw EvalError("derivation cannot have an empty set of outputs, at %1%", posDrvName);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("derivation cannot have an empty set of outputs"),
+                        .nixCode = NixCode { .errPos = posDrvName }
+                    });
         };
 
         try {
@@ -706,18 +759,37 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError("required attribute 'builder' missing, at %1%", posDrvName);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("required attribute 'builder' missing"),
+                .nixCode = NixCode { .errPos = posDrvName }
+            });
+
     if (drv.platform == "")
-        throw EvalError("required attribute 'system' missing, at %1%", posDrvName);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("required attribute 'system' missing"),
+                .nixCode = NixCode { .errPos = posDrvName }
+            });
+
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
-        throw EvalError("derivation names are not allowed to end in '%s', at %s", drvExtension, posDrvName);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("derivation names are not allowed to end in '%s'", drvExtension),
+                .nixCode = NixCode { .errPos = posDrvName }
+            });
+
 
     if (outputHash) {
         /* Handle fixed-output derivations. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            throw Error("multiple outputs are not supported in fixed-output derivations, at %1%", posDrvName);
+            throw Error(
+                ErrorInfo { 
+                    .hint = hintfmt("multiple outputs are not supported in fixed-output derivations"),
+                    .nixCode = NixCode { .errPos = posDrvName }
+                });
 
         HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
         Hash h(*outputHash, ht);
@@ -818,7 +890,11 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
        e.g. nix-push does the right thing. */
     if (!state.store->isStorePath(path)) path = canonPath(path, true);
     if (!state.store->isInStore(path))
-        throw EvalError("path '%1%' is not in the Nix store, at %2%", path, pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("path '%1%' is not in the Nix store", path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     Path path2 = state.store->toStorePath(path);
     if (!settings.readOnlyMode)
         state.store->ensurePath(state.store->parseStorePath(path2));
@@ -835,8 +911,12 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
         throw EvalError(
-                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%", 
+            ErrorInfo {
-                path, e.path, pos);
+                .hint = hintfmt(
+                    "cannot check the existence of '%1%', since path '%2%' is not valid", 
+                    path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 
     try {
@@ -879,8 +959,12 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError("cannot read '%1%', since path '%2%' is not valid, at %3%"
+        throw EvalError(
-           , path, e.path, pos);
+            ErrorInfo {
+                .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid"
+                     , path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
  }
     string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
     if (s.find((char) 0) != string::npos)
@@ -908,7 +992,11 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
 
         i = v2.attrs->find(state.symbols.create("path"));
         if (i == v2.attrs->end())
-            throw EvalError("attribute 'path' missing, at %1%", pos);
+            throw EvalError(
+                ErrorInfo {
+                    .hint = hintfmt("attribute 'path' missing"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
 
         PathSet context;
         string path = state.coerceToString(pos, *i->value, context, false, false);
@@ -916,8 +1004,12 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         try {
             state.realiseContext(context);
         } catch (InvalidPathError & e) {
-            throw EvalError("cannot find '%1%', since path '%2%' is not valid, at %3%",
+            throw EvalError(
-                        path, e.path, pos);
+                ErrorInfo {
+                    .hint = hintfmt("cannot find '%1%', since path '%2%' is not valid",
+                        path, e.path),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         }
 
         searchPath.emplace_back(prefix, path);
@@ -934,7 +1026,11 @@ static void prim_hashFile(EvalState & state, const Pos & pos, Value * * args, Va
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-      throw Error("unknown hash type '%1%', at %2%", type, pos);
+      throw Error(
+          ErrorInfo {
+              .hint = hintfmt("unknown hash type '%1%'", type),
+              .nixCode = NixCode { .errPos = pos }
+          });
 
     PathSet context; // discarded
     Path p = state.coerceToPath(pos, *args[1], context);
@@ -950,8 +1046,12 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
     try {
         state.realiseContext(ctx);
     } catch (InvalidPathError & e) {
-        throw EvalError("cannot read '%1%', since path '%2%' is not valid, at %3%",
+        throw EvalError(
-                        path, e.path, pos);
+            ErrorInfo {
+                .hint = hintfmt("cannot read '%1%', since path '%2%' is not valid",
+                        path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 
     DirEntries entries = readDirectory(state.checkSourcePath(path));
@@ -1021,7 +1121,13 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%", name, pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt(
+                        "in 'toFile': the file '%1%' cannot refer to derivation outputs",  \
+                        name),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1089,11 +1195,21 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
     PathSet context;
     Path path = state.coerceToPath(pos, *args[1], context);
     if (!context.empty())
-        throw EvalError("string '%1%' cannot refer to other paths, at %2%", path, pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("string '%1%' cannot refer to other paths", path),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     state.forceValue(*args[0]);
     if (args[0]->type != tLambda)
-        throw TypeError("first argument in call to 'filterSource' is not a function but %1%, at %2%", showType(*args[0]), pos);
+        throw TypeError(
+            ErrorInfo { 
+                .hint = hintfmt(
+                    "first argument in call to 'filterSource' is not a function but %1%", 
+                    showType(*args[0])),
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], true, Hash(), v);
 }
@@ -1113,7 +1229,13 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
             PathSet context;
             path = state.coerceToPath(*attr.pos, *attr.value, context);
             if (!context.empty())
-                throw EvalError("string '%1%' cannot refer to other paths, at %2%", path, *attr.pos);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("string '%1%' cannot refer to other paths", 
+                            path),
+                        .nixCode = NixCode { .errPos = *attr.pos }
+                    });
+
         } else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, *attr.pos);
         else if (n == "filter") {
@@ -1124,10 +1246,19 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
         else
-            throw EvalError("unsupported argument '%1%' to 'addPath', at %2%", attr.name, *attr.pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("unsupported argument '%1%' to 'addPath'",
+                        attr.name),
+                    .nixCode = NixCode { .errPos = *attr.pos }
+                });
     }
     if (path.empty())
-        throw EvalError("'path' required, at %1%", pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("'path' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     if (name.empty())
         name = baseNameOf(path);
 
@@ -1185,7 +1316,12 @@ void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
     // !!! Should we create a symbol here or just do a lookup?
     Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
     if (i == args[1]->attrs->end())
-        throw EvalError("attribute '%1%' missing, at %2%", attr, pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("attribute '%1%' missing", attr),
+                .nixCode = NixCode { .errPos = pos }
+            });
+
     // !!! add to stack trace?
     if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
     state.forceValue(*i->value);
@@ -1265,14 +1401,23 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
 
         Bindings::iterator j = v2.attrs->find(state.sName);
         if (j == v2.attrs->end())
-            throw TypeError("'name' attribute missing in a call to 'listToAttrs', at %1%", pos);
+            throw TypeError(
+                ErrorInfo { 
+                    .hint = hintfmt("'name' attribute missing in a call to 'listToAttrs'"), 
+                    .nixCode = NixCode { .errPos = pos }
+                });
+
         string name = state.forceStringNoCtx(*j->value, pos);
 
         Symbol sym = state.symbols.create(name);
         if (seen.insert(sym).second) {
             Bindings::iterator j2 = v2.attrs->find(state.symbols.create(state.sValue));
             if (j2 == v2.attrs->end())
-                throw TypeError("'value' attribute missing in a call to 'listToAttrs', at %1%", pos);
+                throw TypeError(
+                    ErrorInfo { 
+                        .hint = hintfmt("'value' attribute missing in a call to 'listToAttrs'"), 
+                        .nixCode = NixCode { .errPos = pos }
+                    });
 
             v.attrs->push_back(Attr(sym, j2->value, j2->pos));
         }
@@ -1346,7 +1491,12 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
 {
     state.forceValue(*args[0]);
     if (args[0]->type != tLambda)
-        throw TypeError("'functionArgs' requires a function, at %1%", pos);
+        throw TypeError(
+            ErrorInfo { 
+                .hint = hintfmt("'functionArgs' requires a function"), 
+                .nixCode = NixCode { .errPos = pos }
+            });
+
 
     if (!args[0]->lambda.fun->matchAttrs) {
         state.mkAttrs(v, 0);
@@ -1396,7 +1546,12 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
 {
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
-        throw Error("list index %1% is out of bounds, at %2%", n, pos);
+        throw Error(
+            ErrorInfo { 
+                .hint = hintfmt("list index %1% is out of bounds", n), 
+                .nixCode = NixCode { .errPos = pos }
+            });
+
     state.forceValue(*list.listElems()[n]);
     v = *list.listElems()[n];
 }
@@ -1423,7 +1578,12 @@ static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error("'tail' called on an empty list, at %1%", pos);
+        throw Error(
+            ErrorInfo { 
+                .hint = hintfmt("'tail' called on an empty list"), 
+                .nixCode = NixCode { .errPos = pos }
+            });
+
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
         v.listElems()[n] = args[0]->listElems()[n + 1];
@@ -1564,7 +1724,12 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError("cannot create list of size %1%, at %2%", len, pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("cannot create list of size %1%", len), 
+                .nixCode = NixCode { .errPos = pos }
+            });
+
 
     state.mkList(v, len);
 
@@ -1722,7 +1887,12 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[1], pos);
 
     NixFloat f2 = state.forceFloat(*args[1], pos);
-    if (f2 == 0) throw EvalError("division by zero, at %1%", pos);
+    if (f2 == 0) 
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("division by zero"), 
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     if (args[0]->type == tFloat || args[1]->type == tFloat) {
         mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
@@ -1731,7 +1901,12 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
         NixInt i2 = state.forceInt(*args[1], pos);
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            throw EvalError("overflow in integer division, at %1%", pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("overflow in integer division"), 
+                    .nixCode = NixCode { .errPos = pos }
+                });
+
         mkInt(v, i1 / i2);
     }
 }
@@ -1787,7 +1962,12 @@ static void prim_substring(EvalState & state, const Pos & pos, Value * * args, V
     PathSet context;
     string s = state.coerceToString(pos, *args[2], context);
 
-    if (start < 0) throw EvalError("negative start position in 'substring', at %1%", pos);
+    if (start < 0) 
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("negative start position in 'substring'"), 
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     mkString(v, (unsigned int) start >= s.size() ? "" : string(s, start, len), context);
 }
@@ -1807,7 +1987,11 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-      throw Error("unknown hash type '%1%', at %2%", type, pos);
+        throw Error(
+            ErrorInfo { 
+                .hint = hintfmt("unknown hash type '%1%'", type), 
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     PathSet context; // discarded
     string s = state.forceString(*args[1], context, pos);
@@ -1850,9 +2034,17 @@ void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
     } catch (std::regex_error &e) {
         if (e.code() == std::regex_constants::error_space) {
             // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-          throw EvalError("memory limit exceeded by regular expression '%s', at %s", re, pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("memory limit exceeded by regular expression '%s'", re), 
+                    .nixCode = NixCode { .errPos = pos }
+                });
         } else {
-          throw EvalError("invalid regular expression '%s', at %s", re, pos);
+          throw EvalError(
+              ErrorInfo { 
+                  .hint = hintfmt("invalid regular expression '%s'", re), 
+                  .nixCode = NixCode { .errPos = pos }
+              });
         }
     }
 }
@@ -1917,9 +2109,17 @@ static void prim_split(EvalState & state, const Pos & pos, Value * * args, Value
     } catch (std::regex_error &e) {
         if (e.code() == std::regex_constants::error_space) {
           // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
-          throw EvalError("memory limit exceeded by regular expression '%s', at %s", re, pos);
+          throw EvalError(
+              ErrorInfo { 
+                  .hint = hintfmt("memory limit exceeded by regular expression '%s'", re), 
+                  .nixCode = NixCode { .errPos = pos }
+              });
          } else {
-          throw EvalError("invalid regular expression '%s', at %s", re, pos);
+          throw EvalError(
+              ErrorInfo { 
+                  .hint = hintfmt("invalid regular expression '%s'", re), 
+                  .nixCode = NixCode { .errPos = pos }
+              });
          }
     }
 }
@@ -1950,7 +2150,11 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     state.forceList(*args[0], pos);
     state.forceList(*args[1], pos);
     if (args[0]->listSize() != args[1]->listSize())
-        throw EvalError("'from' and 'to' arguments to 'replaceStrings' have different lengths, at %1%", pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"), 
+                .nixCode = NixCode { .errPos = pos }
+            });
 
     vector<string> from;
     from.reserve(args[0]->listSize());
@@ -2072,11 +2276,20 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             else if (n == "name")
                 request.name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError("unsupported argument '%1%' to '%2%', at %3%", attr.name, who, attr.pos);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("unsupported argument '%1%' to '%2%'", attr.name, who), 
+                        .nixCode = NixCode { .errPos = pos }
+                    });
+
         }
 
         if (request.uri.empty())
-            throw EvalError("'url' argument required, at %1%", pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("'url' argument required"), 
+                    .nixCode = NixCode { .errPos = pos }
+                });
 
     } else
         request.uri = state.forceStringNoCtx(*args[0], pos);

src/libexpr/primops/context.cc

@@ -146,7 +146,12 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
     auto sAllOutputs = state.symbols.create("allOutputs");
     for (auto & i : *args[1]->attrs) {
         if (!state.store->isStorePath(i.name))
-            throw EvalError("Context key '%s' is not a store path, at %s", i.name, i.pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("Context key '%s' is not a store path", i.name),
+                    .nixCode = NixCode { .errPos = *i.pos }
+                });
+
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(i.name));
         state.forceAttrs(*i.value, *i.pos);
@@ -160,7 +165,12 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (iter != i.value->attrs->end()) {
             if (state.forceBool(*iter->value, *iter->pos)) {
                 if (!isDerivation(i.name)) {
-                    throw EvalError("Tried to add all-outputs context of %s, which is not a derivation, to a string, at %s", i.name, i.pos);
+                    throw EvalError(
+                        ErrorInfo { 
+                            .hint = hintfmt("Tried to add all-outputs context of %s, which is not a derivation, to a string", i.name),
+                            .nixCode = NixCode { .errPos = *i.pos }
+                        });
+
                 }
                 context.insert("=" + string(i.name));
             }
@@ -170,7 +180,11 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
         if (iter != i.value->attrs->end()) {
             state.forceList(*iter->value, *iter->pos);
             if (iter->value->listSize() && !isDerivation(i.name)) {
-                throw EvalError("Tried to add derivation output context of %s, which is not a derivation, to a string, at %s", i.name, i.pos);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("Tried to add derivation output context of %s, which is not a derivation, to a string", i.name),
+                        .nixCode = NixCode { .errPos = *i.pos }
+                    });
             }
             for (unsigned int n = 0; n < iter->value->listSize(); ++n) {
                 auto name = state.forceStringNoCtx(*iter->value->listElems()[n], *iter->pos);

src/libexpr/primops/fetchGit.cc

@@ -217,11 +217,19 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError("unsupported argument '%s' to 'fetchGit', at %s", attr.name, *attr.pos);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("unsupported argument '%s' to 'fetchGit'", attr.name),
+                        .nixCode = NixCode { .errPos = *attr.pos }
+                    });
         }
 
         if (url.empty())
-            throw EvalError("'url' argument required, at %1%", pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("'url' argument required"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);

src/libexpr/primops/fetchMercurial.cc

@@ -187,11 +187,20 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError("unsupported argument '%s' to 'fetchMercurial', at %s", attr.name, *attr.pos);
+                throw EvalError(
+                    ErrorInfo { 
+                        .hint = hintfmt("unsupported argument '%s' to 'fetchMercurial'", attr.name),
+                        .nixCode = NixCode { .errPos = *attr.pos }
+                    });
+
         }
 
         if (url.empty())
-            throw EvalError("'url' argument required, at %1%", pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("'url' argument required"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);

src/libutil/error.cc

@@ -1,4 +1,4 @@
-#include "types.hh"
+#include "error.hh"
 
 #include <iostream>
 #include <optional>
@@ -25,31 +25,36 @@ std::ostream& operator<<(std::ostream &os, const hintformat &hf)
 
 string showErrPos(const ErrPos &errPos)
 {
+    if (errPos.line > 0) {
         if (errPos.column > 0) {
             return fmt("(%1%:%2%)", errPos.line, errPos.column);
         } else {
             return fmt("(%1%)", errPos.line);
-    };
+        }
+    }
+    else {
+        return "";
+    }
 }
 
-void printCodeLines(const string &prefix, const NixCode &nixCode)
+void printCodeLines(std::ostream &out, const string &prefix, const NixCode &nixCode)
 {
     // previous line of code.
     if (nixCode.prevLineOfCode.has_value()) {
-        std::cout << fmt("%1% %|2$5d|| %3%",
+        out << fmt("%1% %|2$5d|| %3%",
             prefix,
             (nixCode.errPos.line - 1),
             *nixCode.prevLineOfCode)
             << std::endl;
     }
 
-    // line of code containing the error.%2$+5d%
+    if (nixCode.errLineOfCode.has_value()) {
-    std::cout << fmt("%1% %|2$5d|| %3%",
+        // line of code containing the error.
+        out << fmt("%1% %|2$5d|| %3%",
             prefix,
             (nixCode.errPos.line),
-        nixCode.errLineOfCode)
+            *nixCode.errLineOfCode)
             << std::endl;
-
         // error arrows for the column range.
         if (nixCode.errPos.column > 0) {
             int start = nixCode.errPos.column;
@@ -60,15 +65,16 @@ void printCodeLines(const string &prefix, const NixCode &nixCode)
 
             std::string arrows("^");
 
-        std::cout << fmt("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL,
+            out << fmt("%1%      |%2%" ANSI_RED "%3%" ANSI_NORMAL,
                 prefix,
                 spaces,
                 arrows) << std::endl;
         }
+    }
 
     // next line of code.
     if (nixCode.nextLineOfCode.has_value()) {
-        std::cout << fmt("%1% %|2$5d|| %3%",
+        out << fmt("%1% %|2$5d|| %3%",
             prefix,
             (nixCode.errPos.line + 1),
             *nixCode.nextLineOfCode)
@@ -158,14 +164,10 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
     // filename.
     if (einfo.nixCode.has_value()) {
         if (einfo.nixCode->errPos.file != "") {
-            string eline = einfo.nixCode->errLineOfCode != ""
-                           ? string(" ") + showErrPos(einfo.nixCode->errPos)
-                           : "";
-
             out << fmt("%1%in file: " ANSI_BLUE "%2% %3%" ANSI_NORMAL,
                 prefix,
                 einfo.nixCode->errPos.file,
-                eline) << std::endl;
+                showErrPos(einfo.nixCode->errPos)) << std::endl;
             out << prefix << std::endl;
         } else {
             out << fmt("%1%from command line argument", prefix) << std::endl;
@@ -180,8 +182,8 @@ std::ostream& operator<<(std::ostream &out, const ErrorInfo &einfo)
     }
 
     // lines of code.
-    if (einfo.nixCode.has_value() && einfo.nixCode->errLineOfCode != "") {
+    if (einfo.nixCode.has_value()) {
-        printCodeLines(prefix, *einfo.nixCode);
+        printCodeLines(out, prefix, *einfo.nixCode);
         out << prefix << std::endl;
     }
 

src/libutil/error.hh

@@ -35,10 +35,15 @@ typedef enum {
 } Verbosity;
 
 struct ErrPos {
-    int line;
+    int line = 0;
-    int column;
+    int column = 0;
     string file;
 
+    operator bool() const
+    {
+        return line != 0;
+    }
+
     template <class P>
     ErrPos& operator=(const P &pos)
     {
@@ -58,7 +63,7 @@ struct ErrPos {
 struct NixCode {
     ErrPos errPos;
     std::optional<string> prevLineOfCode;
-    string errLineOfCode;
+    std::optional<string> errLineOfCode;
     std::optional<string> nextLineOfCode;
 };
 

src/libutil/logging.cc

@@ -171,7 +171,8 @@ struct JSONLogger : Logger {
                 json["file"] = ei.nixCode->errPos.file;
             if (ei.nixCode->prevLineOfCode.has_value())
                 json["prevLineOfCode"] = *ei.nixCode->prevLineOfCode;
-            json["errLineOfCode"] = ei.nixCode->errLineOfCode;
+            if (ei.nixCode->errLineOfCode.has_value())
+                json["errLineOfCode"] = *ei.nixCode->errLineOfCode;
             if (ei.nixCode->nextLineOfCode.has_value())
                 json["nextLineOfCode"] = *ei.nixCode->nextLineOfCode;
         }

tests/misc.sh

@@ -16,4 +16,6 @@ nix-env --foo 2>&1 | grep "no operation"
 nix-env -q --foo 2>&1 | grep "unknown flag"
 
 # Eval Errors.
-nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 | grep "infinite recursion encountered, at .*(string).*:1:15$"
+eval_res=$(nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 || true)
+echo $eval_res | grep "(string) (1:15)"
+echo $eval_res | grep "infinite recursion encountered"