diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index c719f660c..a869cfb97 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -215,9 +215,8 @@ static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positi
         : (std::shared_ptr<AbstractPos>) positions[dt.expr.getPos() ? dt.expr.getPos() : noPos];
 
     if (pos) {
-        pos->print(out);
-
-        if (auto loc = pos->getCodeLines()) {;
+        out << pos;
+        if (auto loc = pos->getCodeLines()) {
             out << "\n";
             printCodeLines(out, "", *pos, *loc);
             out << "\n";
@@ -584,7 +583,9 @@ bool NixRepl::processLine(std::string line)
                 return {filename, 0};
             } else if (v.isLambda()) {
                 auto pos = state->positions[v.lambda.fun->pos];
-                return {pos.file, pos.line};
+                // FIXME
+                abort();
+                //return {pos.file, pos.line};
             } else {
                 // assume it's a derivation
                 return findPackageFilename(*state, v, arg);
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 20afd20fe..89887823e 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1081,9 +1081,9 @@ void EvalState::mkThunk_(Value & v, Expr * expr)
 void EvalState::mkPos(Value & v, PosIdx p)
 {
     auto pos = positions[p];
-    if (!pos.file.empty()) {
+    if (auto path = std::get_if<SourcePath>(&pos.origin)) {
         auto attrs = buildBindings(3);
-        attrs.alloc(sFile).mkString(pos.file);
+        attrs.alloc(sFile).mkString(path->path.abs());
         attrs.alloc(sLine).mkInt(pos.line);
         attrs.alloc(sColumn).mkInt(pos.column);
         v.mkAttrs(attrs);
@@ -1450,7 +1450,8 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
 
     } catch (Error & e) {
         auto pos2r = state.positions[pos2];
-        if (pos2 && pos2r.file != state.derivationNixPath)
+        // FIXME: use MemoryAccessor
+        if (pos2 /* && pos2r.origin != Pos(state.derivationNixPath) */)
             state.addErrorTrace(e, pos2, "while evaluating the attribute '%1%'",
                 showAttrPath(state, env, attrPath));
         throw;
@@ -2465,7 +2466,8 @@ void EvalState::printStats()
                     else
                         obj.attr("name", nullptr);
                     if (auto pos = positions[i.first->pos]) {
-                        obj.attr("file", (const std::string &) pos.file);
+                        // FIXME
+                        //obj.attr("file", (const std::string &) pos.file);
                         obj.attr("line", pos.line);
                         obj.attr("column", pos.column);
                     }
@@ -2477,7 +2479,8 @@ void EvalState::printStats()
                 for (auto & i : attrSelects) {
                     auto obj = list.object();
                     if (auto pos = positions[i.first]) {
-                        obj.attr("file", (const std::string &) pos.file);
+                        // FIXME
+                        //obj.attr("file", (const std::string &) pos.file);
                         obj.attr("line", pos.line);
                         obj.attr("column", pos.column);
                     }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 11038317f..56519573b 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -453,8 +453,7 @@ private:
     Expr * parse(
         char * text,
         size_t length,
-        FileOrigin origin,
-        const PathView path,
+        Pos::Origin origin,
         const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv);
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index f24347665..933dad35a 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -154,7 +154,7 @@ static Flake readFlake(
     Value vInfo;
     state.evalFile(flakePath, vInfo, true);
 
-    expectType(state, nAttrs, vInfo, state.positions.add({flakePath.to_string(), foFile}, 0, 0));
+    expectType(state, nAttrs, vInfo, state.positions.add(Pos::Origin(rootDir), 1, 1));
 
     Flake flake {
         .originalRef = originalRef,
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 8f19b24d0..08478455b 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -10,16 +10,27 @@ namespace nix {
 
 struct SourcePathAdapter : AbstractPos
 {
-    std::string file;
+    SourcePath path;
+    ref<InputAccessor> hack; // FIXME: remove
+
+    SourcePathAdapter(SourcePath path)
+        : path(std::move(path))
+        , hack(path.accessor.shared_from_this())
+    {
+    }
 
     std::optional<std::string> getSource() const override
     {
-        return std::nullopt;
+        try {
+            return path.readFile();
+        } catch (Error &) {
+            return std::nullopt;
+        }
     }
 
     void print(std::ostream & out) const override
     {
-        out << fmt(ANSI_BLUE "at " ANSI_WARNING "%s:%s" ANSI_NORMAL ":", file, showErrPos());
+        out << path;
     }
 };
 
@@ -27,7 +38,7 @@ struct StringPosAdapter : AbstractPos
 {
     void print(std::ostream & out) const override
     {
-        out << fmt(ANSI_BLUE "at " ANSI_WARNING "«string»:%s" ANSI_NORMAL ":", showErrPos());
+        out << "«string»";
     }
 };
 
@@ -35,35 +46,27 @@ struct StdinPosAdapter : AbstractPos
 {
     void print(std::ostream & out) const override
     {
-        out << fmt(ANSI_BLUE "at " ANSI_WARNING "«stdin»:%s" ANSI_NORMAL ":", showErrPos());
+        out << "«stdin»";
     }
 };
 
 Pos::operator std::shared_ptr<AbstractPos>() const
 {
-    if (!line) return nullptr;
+    std::shared_ptr<AbstractPos> pos;
 
-    switch (origin) {
-    case foFile: {
-        auto pos = std::make_shared<SourcePathAdapter>();
+    if (auto path = std::get_if<SourcePath>(&origin))
+        pos = std::make_shared<SourcePathAdapter>(*path);
+    else if (std::get_if<stdin_tag>(&origin))
+        pos = std::make_shared<StdinPosAdapter>();
+    else if (std::get_if<string_tag>(&origin))
+        pos = std::make_shared<StringPosAdapter>();
+
+    if (pos) {
         pos->line = line;
         pos->column = column;
-        pos->file = file;
-        return pos;
     }
-    case foStdin: {
-        auto pos = std::make_shared<StdinPosAdapter>();
-        pos->line = line;
-        pos->column = column;
-        return pos;
-    }
-    case foString:
-        auto pos = std::make_shared<StringPosAdapter>();
-        pos->line = line;
-        pos->column = column;
-        return pos;
-    }
-    assert(false);
+
+    return pos;
 }
 
 /* Displaying abstract syntax trees. */
@@ -306,24 +309,10 @@ void ExprPos::show(const SymbolTable & symbols, std::ostream & str) const
 
 std::ostream & operator << (std::ostream & str, const Pos & pos)
 {
-    if (!pos)
+    if (auto pos2 = (std::shared_ptr<AbstractPos>) pos) {
+        str << *pos2;
+    } else
         str << "undefined position";
-    else
-    {
-        auto f = format(ANSI_BOLD "%1%" ANSI_NORMAL ":%2%:%3%");
-        switch (pos.origin) {
-            case foFile:
-                f % (const std::string &) pos.file;
-                break;
-            case foStdin:
-            case foString:
-                f % "(string)";
-                break;
-            default:
-                throw Error("unhandled Pos origin!");
-        }
-        str << (f % pos.line % pos.column).str();
-    }
 
     return str;
 }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 3fe75e09b..7b732995e 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -21,21 +21,20 @@ MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 
-// FIXME: change this into a variant?
-typedef enum {
-    foFile,
-    foStdin,
-    foString
-} FileOrigin;
-
 /* Position objects. */
 struct Pos
 {
-    std::string file;
-    FileOrigin origin;
     uint32_t line;
     uint32_t column;
 
+    struct no_pos_tag {};
+    struct stdin_tag {};
+    struct string_tag {};
+
+    typedef std::variant<no_pos_tag, stdin_tag, string_tag, SourcePath> Origin;
+
+    Origin origin;
+
     explicit operator bool() const { return line > 0; }
 
     operator std::shared_ptr<AbstractPos>() const;
@@ -68,13 +67,12 @@ public:
         // current origins.back() can be reused or not.
         mutable uint32_t idx = std::numeric_limits<uint32_t>::max();
 
-        explicit Origin(uint32_t idx): idx(idx), file{}, origin{} {}
+        explicit Origin(uint32_t idx): idx(idx), origin{Pos::no_pos_tag()} {}
 
     public:
-        const std::string file;
-        const FileOrigin origin;
+        const Pos::Origin origin;
 
-        Origin(std::string file, FileOrigin origin): file(std::move(file)), origin(origin) {}
+        Origin(Pos::Origin origin): origin(origin) {}
     };
 
     struct Offset {
@@ -114,7 +112,7 @@ public:
             [] (const auto & a, const auto & b) { return a.idx < b.idx; });
         const auto origin = *std::prev(pastOrigin);
         const auto offset = offsets[idx];
-        return {origin.file, origin.origin, offset.line, offset.column};
+        return {offset.line, offset.column, origin.origin};
     }
 };
 
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 1f1d7e7f9..14b712f6f 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -648,30 +648,16 @@ namespace nix {
 Expr * EvalState::parse(
     char * text,
     size_t length,
-    FileOrigin origin,
-    const PathView path,
+    Pos::Origin origin,
     const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv)
 {
     yyscan_t scanner;
-    std::string file;
-    switch (origin) {
-        case foFile:
-            // FIXME: change this to a SourcePath.
-            file = path;
-            break;
-        case foStdin:
-        case foString:
-            file = text;
-            break;
-        default:
-            assert(false);
-    }
     ParseData data {
         .state = *this,
         .symbols = symbols,
         .basePath = basePath,
-        .origin = {file, origin},
+        .origin = {origin},
     };
 
     yylex_init(&scanner);
@@ -713,14 +699,14 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, std::shared_ptr<Sta
     // readFile hopefully have left some extra space for terminators
     buffer.append("\0\0", 2);
     // FIXME: pass SourcePaths
-    return parse(buffer.data(), buffer.size(), foFile, path.path.abs(), path.parent(), staticEnv);
+    return parse(buffer.data(), buffer.size(), Pos::Origin(path), path.parent(), staticEnv);
 }
 
 
 Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath, std::shared_ptr<StaticEnv> & staticEnv)
 {
     s.append("\0\0", 2);
-    return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
+    return parse(s.data(), s.size(), Pos::string_tag(), basePath, staticEnv);
 }
 
 
@@ -736,7 +722,7 @@ Expr * EvalState::parseStdin()
     auto buffer = drainFD(0);
     // drainFD should have left some extra space for terminators
     buffer.append("\0\0", 2);
-    return parse(buffer.data(), buffer.size(), foStdin, "", rootPath(absPath(".")), staticBaseEnv);
+    return parse(buffer.data(), buffer.size(), Pos::stdin_tag(), rootPath(absPath(".")), staticBaseEnv);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index eca9c5903..8f8711e8c 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -368,8 +368,8 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        auto base = state.positions[pos];
-        parsed = state.parseExprFromString(std::move(output), state.rootPath(base.file));
+        //auto base = state.positions[pos];
+        parsed = state.parseExprFromString(std::move(output), state.rootPath("/"));
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "While parsing the output from '%1%'", program);
         throw;
@@ -3979,6 +3979,7 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
+    // FIXME: use corepkgsFS.
     sDerivationNix = symbols.create(derivationNixPath);
     auto vDerivation = allocValue();
     addConstant("derivation", vDerivation);
@@ -3996,7 +3997,7 @@ void EvalState::createBaseEnv()
         // the parser needs two NUL bytes as terminators; one of them
         // is implied by being a C string.
         "\0";
-    eval(parse(code, sizeof(code), foFile, derivationNixPath, rootPath("/"), staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), Pos::string_tag(), rootPath("/"), staticBaseEnv), *vDerivation);
 }
 
 
diff --git a/src/libexpr/value-to-xml.cc b/src/libexpr/value-to-xml.cc
index b6824832d..6f6f4d70e 100644
--- a/src/libexpr/value-to-xml.cc
+++ b/src/libexpr/value-to-xml.cc
@@ -24,7 +24,8 @@ static void printValueAsXML(EvalState & state, bool strict, bool location,
 
 static void posToXML(EvalState & state, XMLAttrs & xmlAttrs, const Pos & pos)
 {
-    xmlAttrs["path"] = pos.file;
+    // FIXME
+    //xmlAttrs["path"] = pos.file;
     xmlAttrs["line"] = (format("%1%") % pos.line).str();
     xmlAttrs["column"] = (format("%1%") % pos.column).str();
 }
diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index 52be6473d..fa825b2f6 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -35,13 +35,13 @@ std::ostream & operator<<(std::ostream & os, const hintformat & hf)
     return os << hf.str();
 }
 
-std::string AbstractPos::showErrPos() const
+std::ostream & operator << (std::ostream & str, const AbstractPos & pos)
 {
-    if (column > 0) {
-        return fmt("%d:%d", line, column);
-    } else {
-        return fmt("%d", line);
-    }
+    pos.print(str);
+    str << ":" << pos.line;
+    if (pos.column > 0)
+        str << ":" << pos.column;
+    return str;
 }
 
 std::optional<LinesOfCode> AbstractPos::getCodeLines() const
@@ -202,8 +202,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
     auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
 
     if (einfo.errPos) {
-        oss << "\n";
-        einfo.errPos->print(oss);
+        oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *einfo.errPos << ANSI_NORMAL << ":";
 
         if (auto loc = einfo.errPos->getCodeLines()) {
             oss << "\n";
@@ -226,8 +225,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
             oss << "\n" << "… " << iter->hint.str() << "\n";
 
             if (iter->pos) {
-                oss << "\n";
-                iter->pos->print(oss);
+                oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *iter->pos << ANSI_NORMAL << ":";
 
                 if (auto loc = iter->pos->getCodeLines()) {
                     oss << "\n";
diff --git a/src/libutil/error.hh b/src/libutil/error.hh
index 487c0c2ec..92d6d3e6e 100644
--- a/src/libutil/error.hh
+++ b/src/libutil/error.hh
@@ -73,11 +73,11 @@ struct AbstractPos
 
     virtual void print(std::ostream & out) const = 0;
 
-    std::string showErrPos() const;
-
     std::optional<LinesOfCode> getCodeLines() const;
 };
 
+std::ostream & operator << (std::ostream & str, const AbstractPos & pos);
+
 void printCodeLines(std::ostream & out,
     const std::string & prefix,
     const AbstractPos & errPos,
diff --git a/tests/function-trace.sh b/tests/function-trace.sh
index 0b7f49d82..b0d6c9d59 100755
--- a/tests/function-trace.sh
+++ b/tests/function-trace.sh
@@ -11,7 +11,7 @@ expect_trace() {
             --expr "$expr" 2>&1 \
             | grep "function-trace" \
             | sed -e 's/ [0-9]*$//'
-    );
+    )
 
     echo -n "Tracing expression '$expr'"
     set +e
@@ -32,40 +32,40 @@ expect_trace() {
 
 # failure inside a tryEval
 expect_trace 'builtins.tryEval (throw "example")' "
-function-trace entered (string):1:1 at
-function-trace entered (string):1:19 at
-function-trace exited (string):1:19 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace entered «string»:1:19 at
+function-trace exited «string»:1:19 at
+function-trace exited «string»:1:1 at
 "
 
 # Missing argument to a formal function
 expect_trace '({ x }: x) { }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a formal function
 expect_trace '({ x }: x) { x = "x"; y = "y"; }' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not enough arguments to a lambda
 expect_trace '(x: y: x + y) 1' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Too many arguments to a lambda
 expect_trace '(x: x) 1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 # Not a function
 expect_trace '1 2' "
-function-trace entered (string):1:1 at
-function-trace exited (string):1:1 at
+function-trace entered «string»:1:1 at
+function-trace exited «string»:1:1 at
 "
 
 set -e
diff --git a/tests/misc.sh b/tests/misc.sh
index 2830856ae..7082f2198 100644
--- a/tests/misc.sh
+++ b/tests/misc.sh
@@ -17,10 +17,10 @@ nix-env -q --foo 2>&1 | grep "unknown flag"
 
 # Eval Errors.
 eval_arg_res=$(nix-instantiate --eval -E 'let a = {} // a; in a.foo' 2>&1 || true)
-echo $eval_arg_res | grep "at «string»:1:15:"
+echo $eval_arg_res | grep "at «string»:1:15"
 echo $eval_arg_res | grep "infinite recursion encountered"
 
 eval_stdin_res=$(echo 'let a = {} // a; in a.foo' | nix-instantiate --eval -E - 2>&1 || true)
-echo $eval_stdin_res | grep "at «stdin»:1:15:"
+echo $eval_stdin_res | grep "at «stdin»:1:15"
 echo $eval_stdin_res | grep "infinite recursion encountered"