haskell: add three new helper functions to the lib module

- sdistTarball transforms a normal Haskell build into one that produces a
   release tarball by running "sdist". For example:

     $ nix-shell -p "haskell.lib.sdistTarball haskellPackages.mtl" --command 'tar tfv $nativeBuildInputs/*tar.gz'

 - buildFromSdist transforms a normal Haskell build into one that compiles the
   package the an sdist release tarball created by Nix. For example:

     $ nix-shell -p "haskell.lib.buildFromSdist haskellPackages.mtl" --command "exit 0"

 - buildStrictly transforms a normal Haskell build into one that (a) compiles
   the source code with "-Wall -Werror" flags and (b) uses as input a locally
   generated sdist release tarball. For example:

     $ nix-shell -p "haskell.lib.buildStrictly haskellPackages.mtl" --command "exit 0"

   This function is particularly useful for continuous Hydra builds of Haskell
   packages.

pkgs/development/haskell-modules/lib.nix

@@ -58,4 +58,23 @@ rec {
   doHyperlinkSource = drv: overrideCabal drv (drv: { hyperlinkSource = true; });
   dontHyperlinkSource = drv: overrideCabal drv (drv: { hyperlinkSource = false; });
 
+  sdistTarball = pkg: pkgs.lib.overrideDerivation pkg (drv: {
+    name = "${drv.pname}-source-${drv.version}";
+    buildPhase = "./Setup sdist";
+    haddockPhase = ":";
+    checkPhase = ":";
+    installPhase = "install -D dist/${drv.pname}-${drv.version}.tar.gz $out/${drv.pname}-${drv.version}.tar.gz";
+    fixupPhase = ":";
+  });
+
+  buildFromSdist = pkg: pkgs.lib.overrideDerivation pkg (drv: {
+    unpackPhase = let src = sdistTarball pkg; tarname = "${pkg.pname}-${pkg.version}"; in ''
+      echo "Source tarball is at ${src}/${tarname}.tar.gz"
+      tar xf ${src}/${tarname}.tar.gz
+      cd ${tarname}
+    '';
+  });
+
+  buildStrictly = pkg: buildFromSdist (appendConfigureFlag pkg "--ghc-option=-Wall --ghc-option=-Werror");
+
 }