Merge branch 'errors-phase-2' of https://github.com/bburdette/nix

15 files changed, 494 insertions, 174 deletions

diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 76d101b98..9a9531a3f 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -19,7 +19,7 @@ static Strings parseAttrPath(const string & s)
             ++i;
             while (1) {
                 if (i == s.end())
-                    throw Error(format("missing closing quote in selection path '%1%'") % s);
+                    throw Error("missing closing quote in selection path '%1%'", s);
                 if (*i == '"') break;
                 cur.push_back(*i++);
             }
@@ -60,11 +60,11 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 
             if (v->type != tAttrs)
                 throw TypeError(
-                    format("the expression selected by the selection path '%1%' should be a set but is %2%")
-                    % attrPath % showType(*v));
-
+                    "the expression selected by the selection path '%1%' should be a set but is %2%",
+                    attrPath,
+                    showType(*v)); 
             if (attr.empty())
-                throw Error(format("empty attribute name in selection path '%1%'") % attrPath);
+                throw Error("empty attribute name in selection path '%1%'", attrPath);
 
             Bindings::iterator a = v->attrs->find(state.symbols.create(attr));
             if (a == v->attrs->end())
@@ -77,9 +77,9 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 
             if (!v->isList())
                 throw TypeError(
-                    format("the expression selected by the selection path '%1%' should be a list but is %2%")
-                    % attrPath % showType(*v));
-
+                    "the expression selected by the selection path '%1%' should be a list but is %2%",
+                    attrPath,
+                    showType(*v)); 
             if (attrIndex >= v->listSize())
                 throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", attrIndex, attrPath);
 
diff --git a/src/libexpr/attr-set.hh b/src/libexpr/attr-set.hh
index 118c7bd5d..f5651891f 100644
--- a/src/libexpr/attr-set.hh
+++ b/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;
     }
 
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index 942cda1ea..eee49e02e 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -7,20 +7,28 @@
 
 namespace nix {
 
-LocalNoInlineNoReturn(void throwEvalError(const char * s, const Pos & pos))
+LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s))
 {
-    throw EvalError(format(s) % pos);
+    throw EvalError(
+        ErrorInfo { 
+            .hint = hintfmt(s),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
 {
-    throw TypeError(format(s) % showType(v));
+    throw TypeError(s, showType(v));
 }
 
 
-LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v, const Pos & pos))
+LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const Value & v))
 {
-    throw TypeError(format(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(pos, "infinite recursion encountered");
 }
 
 
@@ -59,7 +67,7 @@ inline void EvalState::forceAttrs(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
     if (v.type != tAttrs)
-        throwTypeError("value is %1% while a set was expected, at %2%", v, pos);
+        throwTypeError(pos, "value is %1% while a set was expected", v);
 }
 
 
@@ -75,7 +83,7 @@ inline void EvalState::forceList(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
     if (!v.isList())
-        throwTypeError("value is %1% while a list was expected, at %2%", v, pos);
+        throwTypeError(pos, "value is %1% while a list was expected", v);
 }
 
 /* Note: Various places expect the allocated memory to be zeroed. */
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 82eb1582e..7bf25ea17 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -501,52 +501,81 @@ Value & EvalState::getBuiltin(const string & name)
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
 {
-    throw EvalError(format(s) % s2);
+    throw EvalError(s, s2);
 }
 
-LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const Pos & pos))
+LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2))
 {
-    throw EvalError(format(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))
 {
-    throw EvalError(format(s) % s2 % s3);
+    throw EvalError(s, s2, s3);
 }
 
-LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3, const Pos & pos))
+LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s, const string & s2, const string & s3))
 {
-    throw EvalError(format(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))
+LocalNoInlineNoReturn(void throwEvalError(const Pos & p1, const char * s, const Symbol & sym, const Pos & p2))
 {
-    throw EvalError(format(s) % sym % p1 % p2);
+    // p1 is where the error occurred; p2 is a position mentioned in the message.
+    throw EvalError(
+        ErrorInfo { 
+            .hint = hintfmt(s, sym, p2),
+            .nixCode = NixCode { .errPos = p1 }
+        });
 }
 
-LocalNoInlineNoReturn(void throwTypeError(const char * s, const Pos & pos))
+LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s))
 {
-    throw TypeError(format(s) % pos);
+    throw TypeError(
+        ErrorInfo { 
+            .hint = hintfmt(s),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
 {
-    throw TypeError(format(s) % s1);
+    throw TypeError(s, s1);
 }
 
-LocalNoInlineNoReturn(void throwTypeError(const char * s, const ExprLambda & fun, const Symbol & s2, const Pos & pos))
+LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const ExprLambda & fun, const Symbol & s2))
 {
-    throw TypeError(format(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))
+LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s, const string & s1))
 {
-    throw AssertionError(format(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))
+LocalNoInlineNoReturn(void throwUndefinedVarError(const Pos & pos, const char * s, const string & s1))
 {
-    throw UndefinedVarError(format(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))
@@ -614,7 +643,7 @@ inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
             return j->value;
         }
         if (!env->prevWith)
-            throwUndefinedVarError("undefined variable '%1%' at %2%", var.name, var.pos);
+            throwUndefinedVarError(var.pos, "undefined variable '%1%'", var.name);
         for (size_t l = env->prevWith; l; --l, env = env->up) ;
     }
 }
@@ -812,7 +841,7 @@ 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(pos, "value is %1% while a Boolean was expected", v);
     return v.boolean;
 }
 
@@ -926,7 +955,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         Symbol nameSym = state.symbols.create(nameVal.string.s);
         Bindings::iterator j = v.attrs->find(nameSym);
         if (j != v.attrs->end())
-            throwEvalError("dynamic attribute '%1%' at %2% already defined at %3%", nameSym, i.pos, *j->pos);
+            throwEvalError(i.pos, "dynamic attribute '%1%' already defined at %2%", nameSym, *j->pos);
 
         i.valueExpr->setName(nameSym);
         /* Keep sorted order so find can catch duplicates */
@@ -1014,7 +1043,7 @@ 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(pos, "attribute '%1%' missing", name);
             }
             vAttrs = j->value;
             pos2 = j->pos;
@@ -1140,7 +1169,7 @@ 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(pos, "attempt to call something which is not a function but %1%", fun);
 
     ExprLambda & lambda(*fun.lambda.fun);
 
@@ -1168,8 +1197,8 @@ 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%",
-                    lambda, i.name, pos);
+                if (!i.def) throwTypeError(pos, "%1% called without required argument '%2%'",
+                    lambda, i.name);
                 env2.values[displ++] = i.def->maybeThunk(*this, env2);
             } else {
                 attrsUsed++;
@@ -1184,7 +1213,7 @@ 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(pos, "%1% called with unexpected argument '%2%'", lambda, i.name);
             abort(); // can't happen
         }
     }
@@ -1273,7 +1302,7 @@ void ExprAssert::eval(EvalState & state, Env & env, Value & v)
     if (!state.evalBool(env, cond, pos)) {
         std::ostringstream out;
         cond->show(out);
-        throwAssertionError("assertion '%1%' failed at %2%", out.str(), pos);
+        throwAssertionError(pos, "assertion '%1%' failed at %2%", out.str());
     }
     body->eval(state, env, v);
 }
@@ -1425,14 +1454,14 @@ 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(pos, "cannot add %1% to an integer", showType(vTmp));
         } 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(pos, "cannot add %1% to a float", showType(vTmp));
         } else
             s << state.coerceToString(pos, vTmp, context, false, firstType == tString);
     }
@@ -1443,7 +1472,7 @@ 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(pos, "a string that refers to a store path cannot be appended to a path");
         auto path = canonPath(s.str());
         mkPath(v, path.c_str());
     } else
@@ -1492,7 +1521,7 @@ 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(pos, "value is %1% while an integer was expected", v);
     return v.integer;
 }
 
@@ -1503,7 +1532,7 @@ 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(pos, "value is %1% while a float was expected", v);
     return v.fpoint;
 }
 
@@ -1512,7 +1541,7 @@ bool EvalState::forceBool(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
     if (v.type != tBool)
-        throwTypeError("value is %1% while a Boolean was expected, at %2%", v, pos);
+        throwTypeError(pos, "value is %1% while a Boolean was expected", v);
     return v.boolean;
 }
 
@@ -1527,7 +1556,7 @@ void EvalState::forceFunction(Value & v, const Pos & pos)
 {
     forceValue(v, pos);
     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(pos, "value is %1% while a function was expected", v);
 }
 
 
@@ -1536,7 +1565,7 @@ 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(pos, "value is %1% while a string was expected", v);
         else
             throwTypeError("value is %1% while a string was expected", v);
     }
@@ -1565,8 +1594,8 @@ 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%",
-                v.string.s, v.string.context[0], pos);
+            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')", 
+                v.string.s, v.string.context[0]);
         else
             throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
                 v.string.s, v.string.context[0]);
@@ -1622,7 +1651,7 @@ 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(pos, "cannot coerce a set to a string");
         return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
     }
 
@@ -1653,7 +1682,7 @@ string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
         }
     }
 
-    throwTypeError("cannot coerce %1% to a string, at %2%", v, pos);
+    throwTypeError(pos, "cannot coerce %1% to a string", v);
 }
 
 
@@ -1684,7 +1713,7 @@ 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(pos, "string '%1%' doesn't represent an absolute path", path);
     return path;
 }
 
@@ -1891,8 +1920,11 @@ void EvalState::printStats()
 
 string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    throw TypeError(format("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 }
+        });
 }
 
 
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index c34e5c383..85376a08f 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -127,14 +127,14 @@ or          { return OR_KW; }
               try {
                   yylval->n = boost::lexical_cast<int64_t>(yytext);
               } catch (const boost::bad_lexical_cast &) {
-                  throw ParseError(format("invalid integer '%1%'") % yytext);
+                  throw ParseError("invalid integer '%1%'", yytext);
               }
               return INT;
             }
 {FLOAT}     { errno = 0;
               yylval->nf = strtod(yytext, 0);
               if (errno != 0)
-                  throw ParseError(format("invalid float '%1%'") % yytext);
+                  throw ParseError("invalid float '%1%'", yytext);
               return FLOAT;
             }
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 63cbef1dd..91a508305 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -267,8 +267,12 @@ void ExprVar::bindVars(const StaticEnv & env)
     /* Otherwise, the variable must be obtained from the nearest
        enclosing `with'.  If there is no `with', then we can issue an
        "undefined variable" error now. */
-    if (withLevel == -1) throw UndefinedVarError(format("undefined variable '%1%' at %2%") % name % pos);
-
+    if (withLevel == -1) 
+        throw UndefinedVarError(
+            ErrorInfo {
+                .hint = hintfmt("undefined variable '%1%'", name),
+                .nixCode = NixCode { .errPos = pos }
+            });
     fromWith = true;
     this->level = withLevel;
 }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 25798cac6..47d0e85ec 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -2,6 +2,7 @@
 
 #include "value.hh"
 #include "symbol-table.hh"
+#include "error.hh"
 
 #include <map>
 
@@ -235,8 +236,11 @@ struct ExprLambda : Expr
         : pos(pos), arg(arg), matchAttrs(matchAttrs), formals(formals), body(body)
     {
         if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
-            throw ParseError(format("duplicate formal function argument '%1%' at %2%")
-                % arg % pos);
+            throw ParseError(
+                ErrorInfo {
+                    .hint = hintfmt("duplicate formal function argument '%1%'", arg),
+                    .nixCode = NixCode { .errPos = pos }
+                });
     };
     void setName(Symbol & name);
     string showNamePos() const;
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 1993fa6c1..0417a3c21 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -31,7 +31,7 @@ namespace nix {
         Expr * result;
         Path basePath;
         Symbol path;
-        string error;
+        ErrorInfo error;
         Symbol sLetBody;
         ParseData(EvalState & state)
             : state(state)
@@ -64,15 +64,23 @@ namespace nix {
 
 static void dupAttr(const AttrPath & attrPath, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(format("attribute '%1%' at %2% already defined at %3%")
-        % showAttrPath(attrPath) % pos % prevPos);
+    throw ParseError(
+        ErrorInfo {
+            .hint = hintfmt("attribute '%1%' already defined at %2%",
+                showAttrPath(attrPath), prevPos),
+            .nixCode = NixCode { .errPos = pos },
+        });
 }
 
 
 static void dupAttr(Symbol attr, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(format("attribute '%1%' at %2% already defined at %3%")
-        % attr % pos % prevPos);
+    throw ParseError(
+        ErrorInfo {
+            .hint = hintfmt("attribute '%1%' already defined at %2%",
+                attr, prevPos),
+            .nixCode = NixCode { .errPos = pos },
+        });
 }
 
 
@@ -140,8 +148,12 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
 static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
 {
     if (!formals->argNames.insert(formal.name).second)
-        throw ParseError(format("duplicate formal function argument '%1%' at %2%")
-            % formal.name % pos);
+        throw ParseError(
+            ErrorInfo {
+                .hint = hintfmt("duplicate formal function argument '%1%'",
+                    formal.name),
+                .nixCode = NixCode { .errPos = pos },
+            });
     formals->formals.push_front(formal);
 }
 
@@ -249,8 +261,10 @@ static inline Pos makeCurPos(const YYLTYPE & loc, ParseData * data)
 
 void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * error)
 {
-    data->error = (format("%1%, at %2%")
-        % error % makeCurPos(*loc, data)).str();
+    data->error = ErrorInfo { 
+        .hint = hintfmt(error),
+        .nixCode = NixCode { .errPos = makeCurPos(*loc, data) }
+        };
 }
 
 
@@ -327,8 +341,11 @@ expr_function
     { $$ = new ExprWith(CUR_POS, $2, $4); }
   | LET binds IN expr_function
     { if (!$2->dynamicAttrs.empty())
-        throw ParseError(format("dynamic attributes not allowed in let at %1%")
-            % CUR_POS);
+        throw ParseError(
+            ErrorInfo {
+                .hint = hintfmt("dynamic attributes not allowed in let"),
+                .nixCode = NixCode { .errPos = CUR_POS },
+            });
       $$ = new ExprLet($2, $4);
     }
   | expr_if
@@ -405,7 +422,11 @@ 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; }
@@ -475,8 +496,11 @@ attrs
           $$->push_back(AttrName(str->s));
           delete str;
       } else
-          throw ParseError(format("dynamic attributes not allowed in inherit at %1%")
-              % makeCurPos(@2, data));
+          throw ParseError(
+              ErrorInfo {
+                  .hint = hintfmt("dynamic attributes not allowed in inherit"),
+                  .nixCode = NixCode { .errPos = makeCurPos(@2, data) },
+              });
     }
   | { $$ = new AttrPath; }
   ;
@@ -671,11 +695,11 @@ Path EvalState::findFile(SearchPath & searchPath, const string & path, const Pos
         Path res = r.second + suffix;
         if (pathExists(res)) return canonPath(res);
     }
-    format f = format(
-        "file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)"
-        + string(pos ? ", at %2%" : ""));
-    f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
-    throw ThrownError(f % path % pos);
+    throw ThrownError(
+        ErrorInfo { 
+            .hint = hintfmt("file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)", path),
+            .nixCode = NixCode { .errPos = pos }
+        });
 }
 
 
@@ -691,7 +715,11 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
             res = { true, store->toRealPath(fetchers::downloadTarball(
                         store, resolveUri(elem.second), "source", false).storePath) };
         } catch (FileTransferError & e) {
-            printError(format("warning: Nix search path entry '%1%' cannot be downloaded, ignoring") % elem.second);
+            logWarning(
+                ErrorInfo { 
+                    .name = "Entry download",
+                    .hint = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
+            });
             res = { false, "" };
         }
     } else {
@@ -699,7 +727,11 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
         if (pathExists(path))
             res = { true, path };
         else {
-            printError(format("warning: Nix search path entry '%1%' does not exist, ignoring") % elem.second);
+            logWarning(
+                ErrorInfo { 
+                    .name = "Entry not found",
+                    .hint = hintfmt("warning: Nix search path entry '%1%' does not exist, ignoring", elem.second)
+            });
             res = { false, "" };
         }
     }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index eb3116d81..23ab7dec6 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -93,8 +93,12 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError(
+            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));
@@ -170,8 +174,13 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt(
+                    "cannot import '%1%', since path '%2%' is not valid",
+                    path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 
     path = state.checkSourcePath(path);
@@ -180,17 +189,17 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
-        throw EvalError(format("could not open '%1%': %2%") % path % dlerror());
+        throw EvalError("could not open '%1%': %2%", path, dlerror());
 
     dlerror();
     ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
     if(!func) {
         char *message = dlerror();
         if (message)
-            throw EvalError(format("could not load symbol '%1%' from '%2%': %3%") % sym % path % message);
+            throw EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message);
         else
-            throw EvalError(format("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected")
-                    % sym % path);
+            throw EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected",
+                sym, path);
     }
 
     (func)(state, v);
@@ -206,7 +215,11 @@ 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(format("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);
@@ -217,22 +230,25 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot execute '%1%', since path '%2%' is not valid, at %3%")
-            % program % e.path % pos);
-    }
+        throw EvalError(
+            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;
     try {
         parsed = state.parseExprFromString(output, pos.file);
     } catch (Error & e) {
-        e.addPrefix(format("While parsing the output from '%1%', at %2%\n") % program % pos);
+        e.addPrefix(fmt("While parsing the output from '%1%', at %2%\n", program, pos));
         throw;
     }
     try {
         state.eval(parsed, v);
     } catch (Error & e) {
-        e.addPrefix(format("While evaluating the output from '%1%', at %2%\n") % program % pos);
+        e.addPrefix(fmt("While evaluating the output from '%1%', at %2%\n", program, pos));
         throw;
     }
 }
@@ -338,7 +354,7 @@ struct CompareValues
         if (v1->type == tInt && v2->type == tFloat)
             return v1->integer < v2->fpoint;
         if (v1->type != v2->type)
-            throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
+            throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
         switch (v1->type) {
             case tInt:
                 return v1->integer < v2->integer;
@@ -349,7 +365,7 @@ struct CompareValues
             case tPath:
                 return strcmp(v1->path, v2->path) < 0;
             default:
-                throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
+                throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
         }
     }
 };
@@ -370,7 +386,11 @@ 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(format("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;
@@ -381,7 +401,11 @@ 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(format("attribute 'operator' required, at %1%") % pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("attribute 'operator' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     state.forceValue(*op->value, pos);
 
     /* Construct the closure by applying the operator to element of
@@ -400,7 +424,11 @@ 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(format("attribute 'key' required, at %1%") % pos);
+            throw EvalError(
+                ErrorInfo {
+                    .hint = hintfmt("attribute 'key' required"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         state.forceValue(*key->value, pos);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -430,7 +458,7 @@ static void prim_abort(EvalState & state, const Pos & pos, Value * * args, Value
 {
     PathSet context;
     string s = state.coerceToString(pos, *args[0], context);
-    throw Abort(format("evaluation aborted with the following error message: '%1%'") % s);
+    throw Abort("evaluation aborted with the following error message: '%1%'", s);
 }
 
 
@@ -505,9 +533,9 @@ static void prim_trace(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceValue(*args[0], pos);
     if (args[0]->type == tString)
-        printError(format("trace: %1%") % args[0]->string.s);
+        printError("trace: %1%", args[0]->string.s);
     else
-        printError(format("trace: %1%") % *args[0]);
+        printError("trace: %1%", *args[0]);
     state.forceValue(*args[1], pos);
     v = *args[1];
 }
@@ -532,13 +560,17 @@ 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(format("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 {
         drvName = state.forceStringNoCtx(*attr->value, pos);
     } catch (Error & e) {
-        e.addPrefix(format("while evaluating the derivation attribute 'name' at %1%:\n") % posDrvName);
+        e.addPrefix(fmt("while evaluating the derivation attribute 'name' at %1%:\n", posDrvName));
         throw;
     }
 
@@ -575,25 +607,42 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
         auto handleHashMode = [&](const std::string & s) {
             if (s == "recursive") ingestionMethod = FileIngestionMethod::Recursive;
             else if (s == "flat") ingestionMethod = FileIngestionMethod::Flat;
-            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(format("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(format("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(format("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 {
@@ -705,18 +754,35 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError(format("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(format("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(format("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);
 
@@ -821,7 +887,11 @@ 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(format("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));
@@ -837,9 +907,13 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format(
-                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt(
+                    "cannot check the existence of '%1%', since path '%2%' is not valid", 
+                    path, e.path),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 
     try {
@@ -882,12 +956,16 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
-    }
+        throw EvalError(
+            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)
-        throw Error(format("the contents of the file '%1%' cannot be represented as a Nix string") % path);
+        throw Error("the contents of the file '%1%' cannot be represented as a Nix string", path);
     mkString(v, s.c_str());
 }
 
@@ -911,7 +989,11 @@ 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(format("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);
@@ -919,8 +1001,12 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         try {
             state.realiseContext(context);
         } catch (InvalidPathError & e) {
-            throw EvalError(format("cannot find '%1%', since path '%2%' is not valid, at %3%")
-                % path % e.path % pos);
+            throw EvalError(
+                ErrorInfo {
+                    .hint = hintfmt("cannot find '%1%', since path '%2%' is not valid",
+                        path, e.path),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         }
 
         searchPath.emplace_back(prefix, path);
@@ -937,7 +1023,11 @@ 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(format("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);
@@ -953,8 +1043,12 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
     try {
         state.realiseContext(ctx);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError(
+            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));
@@ -1024,9 +1118,15 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError(format(
-                "in 'toFile': the file named '%1%' must not contain a reference "
-                "to a derivation but contains (%2%), at %3%") % name % path % pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt(
+                        "in 'toFile': the file named '%1%' must not contain a reference "
+                        "to a derivation but contains (%2%)",
+                        name,
+                        path),
+                    .nixCode = NixCode { .errPos = pos }
+                });
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1094,11 +1194,21 @@ 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(format("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], pos);
     if (args[0]->type != tLambda)
-        throw TypeError(format("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], FileIngestionMethod::Recursive, Hash(), v);
 }
@@ -1118,7 +1228,12 @@ 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(format("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") {
@@ -1129,10 +1244,19 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
         else
-            throw EvalError(format("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(format("'path' required, at %1%") % pos);
+        throw EvalError(
+            ErrorInfo { 
+                .hint = hintfmt("'path' required"),
+                .nixCode = NixCode { .errPos = pos }
+            });
     if (name.empty())
         name = baseNameOf(path);
 
@@ -1190,7 +1314,11 @@ 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(format("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, pos);
@@ -1270,15 +1398,22 @@ 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(format("'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(format("'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));
         }
     }
@@ -1351,7 +1486,11 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
 {
     state.forceValue(*args[0], pos);
     if (args[0]->type != tLambda)
-        throw TypeError(format("'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);
@@ -1404,7 +1543,11 @@ 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(format("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], pos);
     v = *list.listElems()[n];
 }
@@ -1431,7 +1574,12 @@ static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error(format("'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];
@@ -1572,7 +1720,12 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError(format("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);
 
@@ -1730,7 +1883,12 @@ 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(format("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));
@@ -1739,7 +1897,12 @@ 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(format("overflow in integer division, at %1%") % pos);
+            throw EvalError(
+                ErrorInfo { 
+                    .hint = hintfmt("overflow in integer division"), 
+                    .nixCode = NixCode { .errPos = pos }
+                });
+
         mkInt(v, i1 / i2);
     }
 }
@@ -1795,7 +1958,12 @@ 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(format("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);
 }
@@ -1815,7 +1983,11 @@ 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(format("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);
@@ -1857,10 +2029,18 @@ 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);
+            // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
+            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 }
+                });
         }
     }
 }
@@ -1924,10 +2104,18 @@ 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);
+            // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
+            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 }
+                });
         }
     }
 }
@@ -1958,7 +2146,11 @@ 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(format("'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());
diff --git a/src/libexpr/primops/context.cc b/src/libexpr/primops/context.cc
index 94fa0158c..7f895fc01 100644
--- a/src/libexpr/primops/context.cc
+++ b/src/libexpr/primops/context.cc
@@ -146,7 +146,11 @@ 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 +164,11 @@ 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 +178,11 @@ 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);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 1a8798fcc..52826b56c 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -35,11 +35,19 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
             else if (n == "submodules")
                 fetchSubmodules = state.forceBool(*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(format("'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);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index 0a1ba49d5..bb008ba6b 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -38,11 +38,19 @@ 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(format("'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);
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 97254aa04..62bc4f433 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -66,7 +66,11 @@ static void prim_fetchTree(EvalState & state, const Pos & pos, Value * * args, V
         }
 
         if (!attrs.count("type"))
-            throw Error("attribute 'type' is missing in call to 'fetchTree', at %s", pos);
+            throw Error(
+                ErrorInfo {
+                    .hint = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
+                    .nixCode = NixCode { .errPos = pos }
+                    });
 
         input = fetchers::inputFromAttrs(attrs);
     } else
@@ -107,13 +111,20 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             else if (n == "name")
                 name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError("unsupported argument '%s' to '%s', at %s",
-                    attr.name, who, *attr.pos);
-        }
+                throw EvalError(
+                    ErrorInfo {
+                        .hint = hintfmt("unsupported argument '%s' to '%s'",
+                            attr.name, who),
+                        .nixCode = NixCode { .errPos = *attr.pos }
+                    });
+            }
 
         if (!url)
-            throw EvalError("'url' argument required, at %s", pos);
-
+            throw EvalError(
+                ErrorInfo {
+                    .hint = hintfmt("'url' argument required"),
+                    .nixCode = NixCode { .errPos = pos }
+                });
     } else
         url = state.forceStringNoCtx(*args[0], pos);
 
diff --git a/src/libexpr/primops/fromTOML.cc b/src/libexpr/primops/fromTOML.cc
index c43324dbb..948069401 100644
--- a/src/libexpr/primops/fromTOML.cc
+++ b/src/libexpr/primops/fromTOML.cc
@@ -81,7 +81,11 @@ static void prim_fromTOML(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         visit(v, parser(tomlStream).parse());
     } catch (std::runtime_error & e) {
-        throw EvalError("while parsing a TOML string at %s: %s", pos, e.what());
+        throw EvalError(
+            ErrorInfo {
+                .hint = hintfmt("while parsing a TOML string: %s", e.what()),
+                .nixCode = NixCode { .errPos = pos }
+            });
     }
 }
 
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index 5fe8570ad..6ec8315ba 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -79,7 +79,7 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
 
         default:
-            throw TypeError(format("cannot convert %1% to JSON") % showType(v));
+            throw TypeError("cannot convert %1% to JSON", showType(v));
     }
 }
 
@@ -93,7 +93,7 @@ void printValueAsJSON(EvalState & state, bool strict,
 void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
     JSONPlaceholder & out, PathSet & context) const
 {
-    throw TypeError(format("cannot convert %1% to JSON") % showType());
+    throw TypeError("cannot convert %1% to JSON", showType());
 }