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, 1 insertions, 3 deletions

diff --git a/src/libexpr/common-opts.cc b/src/libexpr/common-opts.cc
index 14a75f7b6f43..a3ea202e889e 100644
--- a/src/libexpr/common-opts.cc
+++ b/src/libexpr/common-opts.cc
@@ -48,9 +48,7 @@ Path lookupFileArg(EvalState & state, string s)
 {
     if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p = s.substr(1, s.size() - 2);
-        Path p2 = state.findFile(p);
-        if (p2 == "") throw Error(format("file `%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)") % p);
-        return p2;
+        return state.findFile(p);
     } else
         return absPath(s);
 }