Merge pull request #15989 from mboes/stack-builder-tweaks

haskell.lib.buildStackProject: allow passing in a custom GHC version.

doc/languages-frameworks/haskell.md

@@ -378,6 +378,23 @@ special options turned on:
 	  buildInputs = [ R zeromq zlib ];
     }
 
+You can select a particular GHC version to compile with by setting the
+`ghc` attribute as an argument to `buildStackProject`. Better yet, let
+Stack choose what GHC version it wants based on the snapshot specified
+in `stack.yaml` (only works with Stack >= 1.1.3):
+
+    {nixpkgs ? import <nixpkgs> { }, ghc ? nixpkgs.ghc}
+
+    with nixpkgs;
+
+    let R = pkgs.R.override { enableStrictBarrier = true; };
+    in
+    haskell.lib.buildStackProject {
+      name = "HaskellR";
+      buildInputs = [ R zeromq zlib ];
+      inherit ghc;
+    }
+
 [stack-nix-doc]: http://docs.haskellstack.org/en/stable/nix_integration.html
 
 ### How to create ad hoc environments for `nix-shell`

pkgs/development/haskell-modules/generic-stack-builder.nix

@@ -4,7 +4,8 @@ with stdenv.lib;
 
 { buildInputs ? []
 , extraArgs ? []
-, LD_LIBRARY_PATH ? ""
+, LD_LIBRARY_PATH ? []
+, ghc ? ghc
 , ...
 }@args:
 
@@ -22,7 +23,7 @@ stdenv.mkDerivation (args // {
     extraArgs;
 
   # XXX: workaround for https://ghc.haskell.org/trac/ghc/ticket/11042.
-  LD_LIBRARY_PATH = "${makeLibraryPath buildInputs}:${LD_LIBRARY_PATH}";
+  LD_LIBRARY_PATH = makeLibraryPath (LD_LIBRARY_PATH ++ buildInputs);
 
   preferLocalBuild = true;