hs.nix.diff

diff --git a/compiler/old-ghc-nix/default.nix b/compiler/old-ghc-nix/default.nix
index 7d8a396..228c329 100644
--- a/compiler/old-ghc-nix/default.nix
+++ b/compiler/old-ghc-nix/default.nix
@@ -3,8 +3,8 @@ let
   fetch = jsonFile:
     with builtins;
     let spec = fromJSON (readFile jsonFile);
-    in pkgs.fetchgit {
+    in builtins.fetchGit {
       name = "old-ghc-nix";
-      inherit (spec) sha256 url rev;
+      inherit (spec) url rev;
     };
 in import (fetch ./old-ghc-nix.json) { inherit pkgs; }
diff --git a/compiler/old-ghc-nix/old-ghc-nix.json b/compiler/old-ghc-nix/old-ghc-nix.json
index bd0c63d..e408881 100644
--- a/compiler/old-ghc-nix/old-ghc-nix.json
+++ b/compiler/old-ghc-nix/old-ghc-nix.json
@@ -1,7 +1,7 @@
 {
   "url": "https://github.com/angerman/old-ghc-nix",
-  "rev": "ee5e7181f681522f075e01a1875228188b03cc8c",
-  "date": "2019-12-08T20:24:29+08:00",
-  "sha256": "0ywvyjvyw9vm0qbf0a7k6zpsrzcz5wa7y16rfcs3wfv36plj3qd5",
+  "rev": "bf640c1a3f55203bb7492a366c6492ff3c211332",
+  "date": "2020-02-27T19:53:36+08:00",
+  "sha256": "050g06911rpmvn66y5lmnszswz17flw3b979imdchc2apji6a1sm",
   "fetchSubmodules": false
 }
diff --git a/lib/materialize.nix b/lib/materialize.nix
index bbf5393..e2ddc46 100644
--- a/lib/materialize.nix
+++ b/lib/materialize.nix
@@ -82,12 +82,10 @@ let
   calculateUseAll = 
     # Skip right to expectedPath if it already exists
     if materialized != null && builtins.pathExists materialized
-      then runCommand name hashArgs ''
-        cp -r ${materialized} $out
-        # Make sure output files can be removed from the sandbox
-        chmod -R +w $out
-      ''
-      else calculateUseHash;
+    then derivation // { outPath = builtins.path {
+      inherit name;
+      path = materialized;
+    }; } else calculateUseHash;
 
 in
   if checkMaterialization