Add primop ‘scopedImport’

‘scopedImport’ works like ‘import’, except that it takes a set of
attributes to be added to the lexical scope of the expression,
essentially extending or overriding the builtin variables.  For
instance, the expression

  scopedImport { x = 1; } ./foo.nix

where foo.nix contains ‘x’, will evaluate to 1.

This has a few applications:

* It allows getting rid of function argument specifications in package
  expressions. For instance, a package expression like:

    { stdenv, fetchurl, libfoo }:

    stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

  can now we written as just

    stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

  and imported in all-packages.nix as:

    bar = scopedImport pkgs ./bar.nix;

  So whereas we once had dependencies listed in three places
  (buildInputs, the function, and the call site), they now only need
  to appear in one place.

* It allows overriding builtin functions. For instance, to trace all
  calls to ‘map’:

  let
    overrides = {
      map = f: xs: builtins.trace "map called!" (map f xs);

      # Ensure that our override gets propagated by calls to
      # import/scopedImport.
      import = fn: scopedImport overrides fn;

      scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;

      # Also update ‘builtins’.
      builtins = builtins // overrides;
    };
  in scopedImport overrides ./bla.nix

* Similarly, it allows extending the set of builtin functions. For
  instance, during Nixpkgs/NixOS evaluation, the Nixpkgs library
  functions could be added to the default scope.

There is a downside: calls to scopedImport are not memoized, unlike
import. So importing a file multiple times leads to multiple parsings
/ evaluations. It would be possible to construct the AST only once,
but that would require careful handling of variables/environments.

src/libexpr/eval.hh

@@ -124,6 +124,7 @@ public:
 
     /* Parse a Nix expression from the specified file. */
     Expr * parseExprFromFile(const Path & path);
+    Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
     Expr * parseExprFromString(const string & s, const Path & basePath, StaticEnv & staticEnv);

src/libexpr/nixexpr.hh

@@ -336,5 +336,4 @@ struct StaticEnv
 };
 
 
-
 }

src/libexpr/parser.y

@@ -592,7 +592,13 @@ Path resolveExprPath(Path path)
 
 Expr * EvalState::parseExprFromFile(const Path & path)
 {
-    return parse(readFile(path).c_str(), path, dirOf(path), staticBaseEnv);
+    return parseExprFromFile(path, staticBaseEnv);
+}
+
+
+Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
+{
+    return parse(readFile(path).c_str(), path, dirOf(path), staticEnv);
 }
 
 
@@ -608,7 +614,7 @@ Expr * EvalState::parseExprFromString(const string & s, const Path & basePath)
 }
 
 
- void EvalState::addToSearchPath(const string & s, bool warn)
+void EvalState::addToSearchPath(const string & s, bool warn)
 {
     size_t pos = s.find('=');
     string prefix;

src/libexpr/primops.cc

@@ -93,6 +93,30 @@ static void prim_import(EvalState & state, const Pos & pos, Value * * args, Valu
 }
 
 
+static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    PathSet context;
+    state.forceAttrs(*args[0]);
+    Path path = resolveExprPath(state.coerceToPath(pos, *args[1], context));
+
+    Env * env = &state.allocEnv(args[0]->attrs->size());
+    env->up = &state.baseEnv;
+
+    StaticEnv staticEnv(false, &state.staticBaseEnv);
+
+    unsigned int displ = 0;
+    for (auto & attr : *args[0]->attrs) {
+        staticEnv.vars[attr.name] = displ;
+        env->values[displ++] = attr.value;
+    }
+
+    startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
+    Expr * e = state.parseExprFromFile(path, staticEnv);
+
+    e->eval(state, *env, v);
+}
+
+
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -1247,6 +1271,7 @@ void EvalState::createBaseEnv()
 
     // Miscellaneous
     addPrimOp("import", 1, prim_import);
+    addPrimOp("scopedImport", 2, prim_scopedImport);
     addPrimOp("__typeOf", 1, prim_typeOf);
     addPrimOp("isNull", 1, prim_isNull);
     addPrimOp("__isFunction", 1, prim_isFunction);

tests/lang/eval-okay-import.exp

@@ -0,0 +1 @@
+[ 1 2 3 4 5 6 7 8 9 10 ]

tests/lang/eval-okay-import.nix

@@ -0,0 +1,11 @@
+let
+
+  overrides = {
+    import = fn: scopedImport overrides fn;
+
+    scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
+
+    builtins = builtins // overrides;
+  } // import ./lib.nix;
+
+in scopedImport overrides ./imported.nix

tests/lang/imported.nix

@@ -0,0 +1,3 @@
+# The function ‘range’ comes from lib.nix and was added to the lexical
+# scope by scopedImport.
+range 1 5 ++ import ./imported2.nix

tests/lang/imported2.nix

@@ -0,0 +1 @@
+range 6 10