Make the Nix search path declarative

Nix search path lookups like <nixpkgs> are now desugared to ‘findFile
nixPath <nixpkgs>’, where ‘findFile’ is a new primop. Thus you can
override the search path simply by saying

  let
    nixPath = [ { prefix = "nixpkgs"; path = "/my-nixpkgs"; } ];
  in ... <nixpkgs> ...

In conjunction with ‘scopedImport’ (commit
c273c15cb13bb86420dda1e5341a4e19517532b5), the Nix search path can be
propagated across imports, e.g.

  let

    overrides = {
      nixPath = [ ... ] ++ builtins.nixPath;
      import = fn: scopedImport overrides fn;
      scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;
      builtins = builtins // overrides;
    };

  in scopedImport overrides ./nixos

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 200ec75e0f..aa706cf983 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -86,6 +86,9 @@ typedef std::map<Path, Path> SrcToStore;
 std::ostream & operator << (std::ostream & str, const Value & v);
 
 
+typedef list<std::pair<string, Path> > SearchPath;
+
+
 class EvalState
 {
 public:
@@ -111,7 +114,6 @@ private:
 #endif
     FileEvalCache fileEvalCache;
 
-    typedef list<std::pair<string, Path> > SearchPath;
     SearchPath searchPath;
 
 public:
@@ -137,6 +139,7 @@ public:
 
     /* Look up a file in the search path. */
     Path findFile(const string & path);
+    Path findFile(SearchPath & searchPath, const string & path);
 
     /* Evaluate an expression to normal form, storing the result in
        value `v'. */