diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 2c62f28d3..80f8986a8 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -451,6 +451,7 @@ EvalState::EvalState(
     , repair(NoRepair)
     , emptyBindings(0)
     , rootFS(makeFSInputAccessor(""))
+    , corepkgsFS(makeMemoryInputAccessor())
     , store(store)
     , buildStore(buildStore ? buildStore : store)
     , regexCache(makeRegexCache())
@@ -500,6 +501,11 @@ EvalState::EvalState(
     }
 
     createBaseEnv();
+
+    corepkgsFS->addFile(
+        "/fetchurl.nix",
+        #include "fetchurl.nix.gen.hh"
+    );
 }
 
 
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 482223036..efc4dd8ec 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -98,6 +98,7 @@ public:
     Bindings emptyBindings;
 
     ref<InputAccessor> rootFS;
+    ref<MemoryInputAccessor> corepkgsFS;
 
     std::unordered_map<size_t, ref<InputAccessor>> inputAccessors;
 
@@ -514,8 +515,6 @@ struct EvalSettings : Config
 
 extern EvalSettings evalSettings;
 
-static const std::string corepkgsPrefix{"/__corepkgs__/"};
-
 }
 
 #include "eval-inline.hh"
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 2d92455b2..bf3240dab 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -788,7 +788,7 @@ Path EvalState::findFile(SearchPath & searchPath, const std::string_view path, c
     }
 
     if (hasPrefix(path, "nix/"))
-        return concatStrings(corepkgsPrefix, path.substr(4));
+        return packPath(SourcePath {corepkgsFS, (std::string) path.substr(3)});
 
     throw ThrownError({
         .msg = hintfmt(evalSettings.pureEval
diff --git a/src/libexpr/paths.cc b/src/libexpr/paths.cc
index 3b19066aa..baf5b24f4 100644
--- a/src/libexpr/paths.cc
+++ b/src/libexpr/paths.cc
@@ -7,6 +7,7 @@ static constexpr std::string_view marker = "/__virtual/";
 
 Path EvalState::packPath(const SourcePath & path)
 {
+    // FIXME: canonPath(path) ?
     printError("PACK %s", path.path);
     assert(hasPrefix(path.path, "/"));
     inputAccessors.emplace(path.accessor->number, path.accessor);
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d238adfdc..ec2060c9d 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -165,15 +165,6 @@ static void import(EvalState & state, const Pos & pos, Value & vPath, Value * vS
 {
     auto path = realisePath(state, pos, vPath);
 
-    #if 0
-    // FIXME: use InputAccessor
-    if (path == corepkgsPrefix + "fetchurl.nix") {
-        state.eval(state.parseExprFromString(
-            #include "fetchurl.nix.gen.hh"
-            , "/"), v);
-    }
-    #endif
-
     state.evalFile(path, v);
 
 #if 0
@@ -216,13 +207,6 @@ static void import(EvalState & state, const Pos & pos, Value & vPath, Value * vS
         state.forceAttrs(v, pos);
     }
 
-    // FIXME: use InputAccessor
-    else if (path == corepkgsPrefix + "fetchurl.nix") {
-        state.eval(state.parseExprFromString(
-            #include "fetchurl.nix.gen.hh"
-            , "/"), v);
-    }
-
     else {
         if (!vScope)
             state.evalFile(path, v);
@@ -1502,7 +1486,6 @@ static RegisterPrimOp primop_readFile({
    which are desugared to 'findFile __nixPath "x"'. */
 static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    #if 0
     state.forceList(*args[0], pos);
 
     SearchPath searchPath;
@@ -1523,7 +1506,18 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
             pos
         );
 
-        auto path = realisePath(state, pos, *i->value, { .requireAbsolutePath = false });
+        PathSet context;
+        auto path = state.coerceToString(pos, *i->value, context, false, false).toOwned();
+
+        try {
+            auto rewrites = state.realiseContext(context);
+            path = rewriteStrings(path, rewrites);
+        } catch (InvalidPathError & e) {
+            throw EvalError({
+                .msg = hintfmt("cannot find '%1%', since path '%2%' is not valid", path, e.path),
+                .errPos = pos
+            });
+        }
 
         searchPath.emplace_back(prefix, path);
     }
@@ -1532,9 +1526,6 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
 
     // FIXME: checkSourcePath?
     v.mkPath(state.findFile(searchPath, path, pos));
-    #endif
-
-    throw UnimplementedError("findFile");
 }
 
 static RegisterPrimOp primop_findFile(RegisterPrimOp::Info {
diff --git a/src/libfetchers/input-accessor.cc b/src/libfetchers/input-accessor.cc
index b548f1baf..19c85bf8c 100644
--- a/src/libfetchers/input-accessor.cc
+++ b/src/libfetchers/input-accessor.cc
@@ -23,7 +23,7 @@ void InputAccessor::dumpPath(
     Sink & sink,
     PathFilter & filter)
 {
-    auto dumpContents = [&](std::string_view path)
+    auto dumpContents = [&](PathView path)
     {
         // FIXME: pipe
         auto s = readFile(path);
@@ -97,7 +97,7 @@ struct FSInputAccessor : InputAccessor
         : root(root)
     { }
 
-    std::string readFile(std::string_view path) override
+    std::string readFile(PathView path) override
     {
         auto absPath = makeAbsPath(path);
         printError("READ %s", absPath);
@@ -105,14 +105,14 @@ struct FSInputAccessor : InputAccessor
         return nix::readFile(absPath);
     }
 
-    bool pathExists(std::string_view path) override
+    bool pathExists(PathView path) override
     {
         auto absPath = makeAbsPath(path);
         printError("EXISTS %s", absPath);
         return isAllowed(absPath) && nix::pathExists(absPath);
     }
 
-    Stat lstat(std::string_view path) override
+    Stat lstat(PathView path) override
     {
         auto absPath = makeAbsPath(path);
         printError("LSTAT %s", absPath);
@@ -128,7 +128,7 @@ struct FSInputAccessor : InputAccessor
         };
     }
 
-    DirEntries readDirectory(std::string_view path) override
+    DirEntries readDirectory(PathView path) override
     {
         auto absPath = makeAbsPath(path);
         printError("READDIR %s", absPath);
@@ -136,7 +136,7 @@ struct FSInputAccessor : InputAccessor
         abort();
     }
 
-    std::string readLink(std::string_view path) override
+    std::string readLink(PathView path) override
     {
         auto absPath = makeAbsPath(path);
         printError("READLINK %s", absPath);
@@ -144,19 +144,19 @@ struct FSInputAccessor : InputAccessor
         return nix::readLink(absPath);
     }
 
-    Path makeAbsPath(std::string_view path)
+    Path makeAbsPath(PathView path)
     {
         assert(hasPrefix(path, "/"));
         return canonPath(root + std::string(path));
     }
 
-    void checkAllowed(std::string_view absPath)
+    void checkAllowed(PathView absPath)
     {
         if (!isAllowed(absPath))
             throw Error("access to path '%s' is not allowed", absPath);
     }
 
-    bool isAllowed(std::string_view absPath)
+    bool isAllowed(PathView absPath)
     {
         if (!isDirOrInDir(absPath, root))
             return false;
@@ -175,4 +175,48 @@ std::ostream & operator << (std::ostream & str, const SourcePath & path)
     return str;
 }
 
+struct MemoryInputAccessorImpl : MemoryInputAccessor
+{
+    std::map<Path, std::string> files;
+
+    std::string readFile(PathView path) override
+    {
+        auto i = files.find((Path) path);
+        if (i == files.end())
+            throw Error("file '%s' does not exist", path);
+        return i->second;
+    }
+
+    bool pathExists(PathView path) override
+    {
+        auto i = files.find((Path) path);
+        return i != files.end();
+    }
+
+    Stat lstat(PathView path) override
+    {
+        throw UnimplementedError("MemoryInputAccessor::lstat");
+    }
+
+    DirEntries readDirectory(PathView path) override
+    {
+        return {};
+    }
+
+    std::string readLink(PathView path) override
+    {
+        throw UnimplementedError("MemoryInputAccessor::readLink");
+    }
+
+    void addFile(PathView path, std::string && contents) override
+    {
+        files.emplace(path, std::move(contents));
+    }
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor()
+{
+    return make_ref<MemoryInputAccessorImpl>();
+}
+
 }
diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index d2fe9c4ed..d4260518a 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -15,9 +15,9 @@ struct InputAccessor
     virtual ~InputAccessor()
     { }
 
-    virtual std::string readFile(std::string_view path) = 0;
+    virtual std::string readFile(PathView path) = 0;
 
-    virtual bool pathExists(std::string_view path) = 0;
+    virtual bool pathExists(PathView path) = 0;
 
     enum Type { tRegular, tSymlink, tDirectory, tMisc };
 
@@ -28,15 +28,15 @@ struct InputAccessor
         bool isExecutable = false; // regular files only
     };
 
-    virtual Stat lstat(std::string_view path) = 0;
+    virtual Stat lstat(PathView path) = 0;
 
     typedef std::optional<Type> DirEntry;
 
     typedef std::map<std::string, DirEntry> DirEntries;
 
-    virtual DirEntries readDirectory(std::string_view path) = 0;
+    virtual DirEntries readDirectory(PathView path) = 0;
 
-    virtual std::string readLink(std::string_view path) = 0;
+    virtual std::string readLink(PathView path) = 0;
 
     virtual void dumpPath(
         const Path & path,
@@ -46,6 +46,13 @@ struct InputAccessor
 
 ref<InputAccessor> makeFSInputAccessor(const Path & root);
 
+struct MemoryInputAccessor : InputAccessor
+{
+    virtual void addFile(PathView path, std::string && contents) = 0;
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor();
+
 struct SourcePath
 {
     ref<InputAccessor> accessor;