haskellPackages.ghc-mod: add overrides to support ghc8

pkgs/development/haskell-modules/configuration-common.nix

@@ -755,7 +755,7 @@ self: super: {
   lens-aeson = dontCheck super.lens-aeson;
 
   # Byte-compile elisp code for Emacs.
-  ghc-mod = overrideCabal (super.ghc-mod.override { cabal-helper = self.cabal-helper_0_6_3_1; }) (drv: {
+  ghc-mod = overrideCabal super.ghc-mod (drv: {
     preCheck = "export HOME=$TMPDIR";
     testToolDepends = drv.testToolDepends or [] ++ [self.cabal-install];
     doCheck = false;            # https://github.com/kazu-yamamoto/ghc-mod/issues/335

pkgs/development/haskell-modules/configuration-ghc-7.10.x.nix

@@ -210,4 +210,6 @@ self: super: {
   semigroups = addBuildDepends super.semigroups (with self; [hashable tagged text unordered-containers]);
   intervals = addBuildDepends super.intervals (with self; [doctest QuickCheck]);
 
+  # Moved out from common as no longer the case for GHC8
+  ghc-mod = super.ghc-mod.override { cabal-helper = self.cabal-helper_0_6_3_1; };
 }

pkgs/development/haskell-modules/configuration-ghc-8.0.x.nix

@@ -53,4 +53,13 @@ self: super: {
     license = pkgs.stdenv.lib.licenses.bsd3;
   }) {};
 
+  # ghc-mod has a ghc-8 branch that has not yet been merged
+  ghc-mod = super."ghc-mod".overrideDerivation (attrs: rec {
+    src = pkgs.fetchFromGitHub {
+      owner  = "DanielG";
+      repo   = "ghc-mod";
+      rev    = "f2c7b01e372dd8c516b1ccbe5a1025cc7814347c";
+      sha256 = "1i45196qrzlhgbisnvkzni4n54saky0i1kyla162xcb5cg3kf2ji";
+    };
+  });
 }