diff --git a/src/libcmd/common-eval-args.cc b/src/libcmd/common-eval-args.cc
index 40435f710..6651b0da1 100644
--- a/src/libcmd/common-eval-args.cc
+++ b/src/libcmd/common-eval-args.cc
@@ -81,7 +81,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
     for (auto & i : autoArgs) {
         auto v = state.allocValue();
         if (i.second[0] == 'E')
-            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), absPath(".")));
+            state.mkThunk_(*v, state.parseExprFromString(i.second.substr(1), state.rootPath(absPath("."))));
         else
             v->mkString(((std::string_view) i.second).substr(1));
         res.insert(state.symbols.create(i.first), v);
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index acd3fbfee..b7494c222 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -766,7 +766,7 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
         else if (file)
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
-            auto e = state->parseExprFromString(*expr, absPath("."));
+            auto e = state->parseExprFromString(*expr, state->rootPath(absPath(".")));
             state->eval(e, *vFile);
         }
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 6d7a9cb12..7d4d870f7 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -190,8 +190,8 @@ public:
     Expr * parseExprFromFile(const SourcePath & path, StaticEnv & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv);
-    Expr * parseExprFromString(std::string s, const Path & basePath);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath, StaticEnv & staticEnv);
+    Expr * parseExprFromString(std::string s, const SourcePath & basePath);
 
     Expr * parseStdin();
 
@@ -356,8 +356,13 @@ private:
     friend struct ExprAttrs;
     friend struct ExprLet;
 
-    Expr * parse(char * text, size_t length, FileOrigin origin, const PathView path,
-        const PathView basePath, StaticEnv & staticEnv);
+    Expr * parse(
+        char * text,
+        size_t length,
+        FileOrigin origin,
+        const PathView path,
+        const SourcePath & basePath,
+        StaticEnv & staticEnv);
 
 public:
 
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index d1a4c0402..424e6ac26 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -723,7 +723,7 @@ void callFlake(EvalState & state,
         state.vCallFlake = allocRootValue(state.allocValue());
         state.eval(state.parseExprFromString(
             #include "call-flake.nix.gen.hh"
-            , "/"), **state.vCallFlake);
+            , state.rootPath("/")), **state.vCallFlake);
     }
 
     state.callFunction(**state.vCallFlake, *vLocks, *vTmp1, noPos);
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index f9d75bf40..953e97419 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -185,10 +185,10 @@ struct ExprPath : Expr
 {
     std::string s; // FIXME: remove
     Value v;
-    ExprPath(InputAccessor & accessor, std::string s)
-        : s(std::move(s))
+    ExprPath(SourcePath && path)
+        : s(path.path)
     {
-        v.mkPath({accessor, this->s});
+        v.mkPath(std::move(path));
     }
     COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 6ede0d27a..4c10d137d 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -31,14 +31,9 @@ namespace nix {
         EvalState & state;
         SymbolTable & symbols;
         Expr * result;
-        Path basePath;
+        SourcePath basePath;
         PosTable::Origin origin;
         std::optional<ErrorInfo> error;
-        ParseData(EvalState & state, PosTable::Origin origin)
-            : state(state)
-            , symbols(state.symbols)
-            , origin(std::move(origin))
-            { };
     };
 
     struct ParserFormals {
@@ -513,15 +508,15 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, data->basePath));
+    SourcePath path { data->basePath.accessor, absPath({$1.p, $1.l}, data->basePath.path) };
     /* add back in the trailing '/' to the first segment */
     if ($1.p[$1.l-1] == '/' && $1.l > 1)
-      path += "/";
-    $$ = new ExprPath(*data->state.rootFS, path);
+      path.path += "/";
+    $$ = new ExprPath(std::move(path));
   }
   | HPATH {
     Path path(getHome() + std::string($1.p + 1, $1.l - 1));
-    $$ = new ExprPath(*data->state.rootFS, path);
+    $$ = new ExprPath(data->state.rootPath(path));
   }
   ;
 
@@ -643,12 +638,13 @@ namespace nix {
 
 
 Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
-    const PathView path, const PathView basePath, StaticEnv & staticEnv)
+    const PathView path, const SourcePath & basePath, StaticEnv & staticEnv)
 {
     yyscan_t scanner;
     std::string file;
     switch (origin) {
         case foFile:
+            // FIXME: change this to a SourcePath.
             file = path;
             break;
         case foStdin:
@@ -658,8 +654,12 @@ Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
         default:
             assert(false);
     }
-    ParseData data(*this, {file, origin});
-    data.basePath = basePath;
+    ParseData data {
+        .state = *this,
+        .symbols = symbols,
+        .basePath = basePath,
+        .origin = {file, origin},
+    };
 
     yylex_init(&scanner);
     yy_scan_buffer(text, length, scanner);
@@ -716,18 +716,18 @@ Expr * EvalState::parseExprFromFile(const SourcePath & path, StaticEnv & staticE
     // 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, dirOf(path.path), staticEnv);
+    return parse(buffer.data(), buffer.size(), foFile, path.path, path.parent(), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath, StaticEnv & staticEnv)
 {
     s.append("\0\0", 2);
     return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
+Expr * EvalState::parseExprFromString(std::string s, const SourcePath & basePath)
 {
     return parseExprFromString(std::move(s), basePath, staticBaseEnv);
 }
@@ -739,7 +739,7 @@ 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, "", absPath("."), staticBaseEnv);
+    return parse(buffer.data(), buffer.size(), foStdin, "", rootPath(absPath(".")), staticBaseEnv);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 6a45278e6..eb973b2ec 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -375,7 +375,7 @@ void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v)
     Expr * parsed;
     try {
         auto base = state.positions[pos];
-        parsed = state.parseExprFromString(std::move(output), base.file);
+        parsed = state.parseExprFromString(std::move(output), state.rootPath(base.file));
     } catch (Error & e) {
         e.addTrace(state.positions[pos], "While parsing the output from '%1%'", program);
         throw;
@@ -1464,7 +1464,7 @@ static void prim_dirOf(EvalState & state, const PosIdx pos, Value * * args, Valu
     state.forceValue(*args[0], pos);
     if (args[0]->type() == nPath) {
         auto path = args[0]->path();
-        v.mkPath({path.accessor, dirOf(path.path)});
+        v.mkPath(path.parent());
     } else {
         auto path = state.coerceToString(pos, *args[0], context, false, false);
         auto dir = dirOf(*path);
@@ -3967,7 +3967,7 @@ 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, "/", staticBaseEnv), *vDerivation);
+    eval(parse(code, sizeof(code), foFile, derivationNixPath, rootPath("/"), staticBaseEnv), *vDerivation);
 }
 
 
diff --git a/src/libfetchers/input-accessor.cc b/src/libfetchers/input-accessor.cc
index 025114515..71eebcdbf 100644
--- a/src/libfetchers/input-accessor.cc
+++ b/src/libfetchers/input-accessor.cc
@@ -87,6 +87,11 @@ void InputAccessor::dumpPath(
     dump(path);
 }
 
+std::string InputAccessor::showPath(PathView path)
+{
+    return "/virtual/" + std::to_string(number) + path;
+}
+
 struct FSInputAccessorImpl : FSInputAccessor
 {
     Path root;
@@ -210,6 +215,11 @@ struct FSInputAccessorImpl : FSInputAccessor
     {
         return (bool) allowedPaths;
     }
+
+    std::string showPath(PathView path) override
+    {
+        return root + path;
+    }
 };
 
 ref<FSInputAccessor> makeFSInputAccessor(
@@ -219,11 +229,6 @@ ref<FSInputAccessor> makeFSInputAccessor(
     return make_ref<FSInputAccessorImpl>(root, std::move(allowedPaths));
 }
 
-std::string SourcePath::to_string() const
-{
-    return path; // FIXME
-}
-
 std::ostream & operator << (std::ostream & str, const SourcePath & path)
 {
     str << path.to_string();
@@ -286,4 +291,10 @@ std::string_view SourcePath::baseName() const
     return path == "" || path == "/" ? "source" : baseNameOf(path);
 }
 
+SourcePath SourcePath::parent() const
+{
+    // FIXME:
+    return {accessor, dirOf(path)};
+}
+
 }
diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index ffa06ecd8..d7fc3f11d 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -52,6 +52,8 @@ struct InputAccessor
     {
         return number < x.number;
     }
+
+    virtual std::string showPath(PathView path);
 };
 
 struct FSInputAccessor : InputAccessor
@@ -87,6 +89,8 @@ struct SourcePath
 
     std::string_view baseName() const;
 
+    SourcePath parent() const;
+
     std::string readFile() const
     { return accessor.readFile(path); }
 
@@ -104,7 +108,8 @@ struct SourcePath
         PathFilter & filter = defaultPathFilter) const
     { return accessor.dumpPath(path, sink, filter); }
 
-    std::string to_string() const;
+    std::string to_string() const
+    { return accessor.showPath(path); }
 
     SourcePath append(std::string_view s) const;
 
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index 384a2bc76..bfe53bb5e 100644
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -292,7 +292,7 @@ static void main_nix_build(int argc, char * * argv)
     else
         for (auto i : left) {
             if (fromArgs)
-                exprs.push_back(state->parseExprFromString(std::move(i), absPath(".")));
+                exprs.push_back(state->parseExprFromString(std::move(i), state->rootPath(absPath("."))));
             else {
                 auto absolute = i;
                 try {
@@ -362,7 +362,9 @@ static void main_nix_build(int argc, char * * argv)
         if (!shell) {
 
             try {
-                auto expr = state->parseExprFromString("(import <nixpkgs> {}).bashInteractive", absPath("."));
+                auto expr = state->parseExprFromString(
+                    "(import <nixpkgs> {}).bashInteractive",
+                    state->rootPath(absPath(".")));
 
                 Value v;
                 state->eval(expr, v);
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 05f0ae20a..21464440d 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -410,7 +410,7 @@ static void queryInstSources(EvalState & state,
             loadSourceExpr(state, instSource.nixExprPath, vArg);
 
             for (auto & i : args) {
-                Expr * eFun = state.parseExprFromString(i, absPath("."));
+                Expr * eFun = state.parseExprFromString(i, state.rootPath(absPath(".")));
                 Value vFun, vTmp;
                 state.eval(eFun, vFun);
                 vTmp.mkApp(&vFun, &vArg);
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index 0b65f1d11..d202c82d4 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -114,7 +114,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
     Value envBuilder;
     state.eval(state.parseExprFromString(
         #include "buildenv.nix.gen.hh"
-            , "/"), envBuilder);
+            , state.rootPath("/")), envBuilder);
 
     /* Construct a Nix expression that calls the user environment
        builder with the manifest as argument. */
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 614201bbb..261760a32 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -182,7 +182,7 @@ static int main_nix_instantiate(int argc, char * * argv)
 
         for (auto & i : files) {
             Expr * e = fromArgs
-                ? state->parseExprFromString(i, absPath("."))
+                ? state->parseExprFromString(i, state->rootPath(absPath(".")))
                 : state->parseExprFromFile(resolveExprPath(lookupFileArg(*state, i)));
             processExpr(*state, attrPaths, parseOnly, strict, autoArgs,
                 evalOnly, outputKind, xmlOutputSourceLocation, e);
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 967dc8519..7f3be0127 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -65,7 +65,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         if (apply) {
             auto vApply = state->allocValue();
-            state->eval(state->parseExprFromString(*apply, absPath(".")), *vApply);
+            state->eval(state->parseExprFromString(*apply, state->rootPath(absPath("."))), *vApply);
             auto vRes = state->allocValue();
             state->callFunction(*vApply, *v, *vRes, noPos);
             v = vRes;
diff --git a/src/nix/main.cc b/src/nix/main.cc
index fe78fe6af..22bc79156 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -176,7 +176,7 @@ static void showHelp(std::vector<std::string> subcommand, MultiCommand & topleve
     auto vGenerateManpage = state.allocValue();
     state.eval(state.parseExprFromString(
         #include "generate-manpage.nix.gen.hh"
-        , "/"), *vGenerateManpage);
+        , state.rootPath("/")), *vGenerateManpage);
 
     state.corepkgsFS->addFile(
         "/utils.nix",
diff --git a/src/nix/prefetch.cc b/src/nix/prefetch.cc
index 73d0cc23e..e983d237d 100644
--- a/src/nix/prefetch.cc
+++ b/src/nix/prefetch.cc
@@ -27,7 +27,10 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
 
     Value vMirrors;
     // FIXME: use nixpkgs flake
-    state.eval(state.parseExprFromString("import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>", "."), vMirrors);
+    state.eval(state.parseExprFromString(
+            "import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>",
+            state.rootPath(absPath("/"))),
+        vMirrors);
     state.forceAttrs(vMirrors, noPos);
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 9dcc9b251..c2b8449ef 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -726,7 +726,7 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
 
 Expr * NixRepl::parseString(std::string s)
 {
-    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
+    Expr * e = state->parseExprFromString(std::move(s), state->rootPath(curDir), staticEnv);
     return e;
 }
 
diff --git a/src/nix/upgrade-nix.cc b/src/nix/upgrade-nix.cc
index 17a5a77ee..e90034699 100644
--- a/src/nix/upgrade-nix.cc
+++ b/src/nix/upgrade-nix.cc
@@ -140,7 +140,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
 
         auto state = std::make_unique<EvalState>(Strings(), store);
         auto v = state->allocValue();
-        state->eval(state->parseExprFromString(res.data, "/no-such-path"), *v);
+        state->eval(state->parseExprFromString(res.data, state->rootPath("/no-such-path")), *v);
         Bindings & bindings(*state->allocBindings(0));
         auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v).first;