From aeadb1644463c8c17e99f4dadb2d86725ea70953 Mon Sep 17 00:00:00 2001
From: Peter Simons <simons@cryp.to>
Date: Sun, 22 Feb 2015 20:43:40 +0100
Subject: [PATCH] ghcWithPackages: don't re-generate the packages.conf.d cache
 if there are no actual libraries added to the environment

This change resolves https://github.com/NixOS/nixpkgs/issues/6419.
---
 pkgs/development/haskell-modules/with-packages-wrapper.nix | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pkgs/development/haskell-modules/with-packages-wrapper.nix b/pkgs/development/haskell-modules/with-packages-wrapper.nix
index 2824296c08f..763164cec87 100644
--- a/pkgs/development/haskell-modules/with-packages-wrapper.nix
+++ b/pkgs/development/haskell-modules/with-packages-wrapper.nix
@@ -30,8 +30,8 @@ let
   libDir        = "$out/lib/ghc-${ghc.version}";
   docDir        = "$out/share/doc/ghc/html";
   packageCfgDir = "${libDir}/package.conf.d";
-  isHaskellPkg  = x: (x ? pname) && (x ? version) && (x ? env);
-  paths         = stdenv.lib.filter isHaskellPkg (stdenv.lib.closePropagation packages);
+  paths         = stdenv.lib.filter (x: x ? isHaskellLibrary) (stdenv.lib.closePropagation packages);
+  hasLibraries  = stdenv.lib.any (x: x.isHaskellLibrary) paths;
 in
 if paths == [] then ghc else
 buildEnv {
@@ -73,7 +73,7 @@ buildEnv {
       makeWrapper ${ghc}/bin/$prg $out/bin/$prg --add-flags "${packageDBFlag}=${packageCfgDir}"
     done
 
-    $out/bin/ghc-pkg recache
+    ${stdenv.lib.optionalString hasLibraries "$out/bin/ghc-pkg recache"}
     $out/bin/ghc-pkg check
   '';
 } // {