Work on Values instead of Exprs

This prevents some duplicate evaluation in nix-env and
nix-instantiate.

Also, when traversing ~/.nix-defexpr, only read regular files with the
extension .nix.  Previously it was reading files like
.../channels/binary-caches/<name>.  The only reason this didn't cause
problems is pure luck (namely, <name> shadows an actual Nix
expression, the binary-caches files happen to be syntactically valid
Nix expressions, and we iterate over the directory contents in just
the right order).

1 files changed, 4 insertions, 2 deletions

diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 0f0710d680..fd2c04eae4 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -44,9 +44,11 @@ void processExpr(EvalState & state, const Strings & attrPaths,
         return;
     }
 
+    Value vRoot;
+    state.eval(e, vRoot);
+
     foreach (Strings::const_iterator, i, attrPaths) {
-        Value v;
-        findAlongAttrPath(state, *i, autoArgs, e, v);
+        Value & v(*findAlongAttrPath(state, *i, autoArgs, vRoot));
         state.forceValue(v);
 
         PathSet context;