Patch ghc-paths to interact better with ghcWithPackages.

When the ghc-paths library is compiled, the paths of the
compiler it is compiled with are being hardcoded in the
library (and can then be queried from other applications
using the library).

But on Nix, packages are compiled with ghc-wrapper, and
subsequently possibly used with a special version of ghc
generated for a particular environment of packages. So
one version of ghc-paths may potentially end up being
used by lots of different instances of ghc. The hardcoding
approach fails.

As a work-around, we now patch ghc-paths so that it allows
setting the paths that can be queried via environment
variables. Specific GHC environments can then set these
environment variables in the wrapper shell script that
invokes GHC.

This should at least partially solve issue #213.

pkgs/development/compilers/ghc/with-packages.nix

@@ -79,7 +79,12 @@ stdenv.mkDerivation rec {
     echo -n "Generating wrappers "
 
     for prg in ghc ghci ghc-${ghc.version} ghci-${ghc.version}; do
-      makeWrapper ${ghc}/bin/$prg $out/bin/$prg --add-flags "-B$linkedTopDir"
+      # The NIX env-vars are picked up by our patched version of ghc-paths.
+      makeWrapper ${ghc}/bin/$prg $out/bin/$prg \
+        --add-flags "-B$linkedTopDir" \
+        --set "NIX_GHC"        "$out/bin/ghc"     \
+        --set "NIX_GHCPKG"     "$out/bin/ghc-pkg" \
+        --set "NIX_GHC_LIBDIR" "$linkedTopDir"
       echo -n .
     done
 

pkgs/development/libraries/haskell/ghc-paths/default.nix

@@ -4,6 +4,7 @@ cabal.mkDerivation (self: {
   pname = "ghc-paths";
   version = "0.1.0.9";
   sha256 = "0ibrr1dxa35xx20cpp8jzgfak1rdmy344dfwq4vlq013c6w8z9mg";
+  patches = [ ./ghc-paths-nix.patch ];
   meta = {
     description = "Knowledge of GHC's installation directories";
     license = self.stdenv.lib.licenses.bsd3;

pkgs/development/libraries/haskell/ghc-paths/ghc-paths-nix.patch

@@ -0,0 +1,32 @@
+diff -Naur ghc-paths-0.1.0.9/GHC/Paths.hs ghc-paths-0.1.0.9-new/GHC/Paths.hs
+--- ghc-paths-0.1.0.9/GHC/Paths.hs	2012-12-16 13:53:45.720148396 +0100
++++ ghc-paths-0.1.0.9-new/GHC/Paths.hs	2012-12-16 13:51:50.073070123 +0100
+@@ -4,10 +4,24 @@
+         ghc, ghc_pkg, libdir, docdir
+   ) where
+
++import Data.Maybe
++import System.Environment
++import System.IO.Unsafe
++
++nixLibdir, nixDocdir, nixGhc, nixGhcPkg :: Maybe FilePath
++nixLibdir = unsafePerformIO (lookupEnv "NIX_GHC_LIBDIR")
++nixDocdir = unsafePerformIO (lookupEnv "NIX_GHC_DOCDIR")
++nixGhc    = unsafePerformIO (lookupEnv "NIX_GHC")
++nixGhcPkg = unsafePerformIO (lookupEnv "NIX_GHCPKG")
++{-# NOINLINE nixLibdir #-}
++{-# NOINLINE nixDocdir #-}
++{-# NOINLINE nixGhc    #-}
++{-# NOINLINE nixGhcPkg #-}
++
+ libdir, docdir, ghc, ghc_pkg :: FilePath
+
+-libdir  = GHC_PATHS_LIBDIR
+-docdir  = GHC_PATHS_DOCDIR
++libdir  = fromMaybe GHC_PATHS_LIBDIR  nixLibdir
++docdir  = fromMaybe GHC_PATHS_DOCDIR  nixDocdir
+
+-ghc     = GHC_PATHS_GHC
+-ghc_pkg = GHC_PATHS_GHC_PKG
++ghc     = fromMaybe GHC_PATHS_GHC     nixGhc
++ghc_pkg = fromMaybe GHC_PATHS_GHC_PKG nixGhcPkg