Add haskell.lib.setBuiltTarget, and support non library compiling of a single target

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

···
   installPhase = ''
     runHook preInstall
 
-    ${if !isLibrary then "${setupCommand} install" else ''
+    ${if !isLibrary && buildTarget == "" then "${setupCommand} install"
+      # ^^ if the project is not a library, and no build target is specified, we can just use "install".
+      else if !isLibrary then "${setupCommand} copy ${buildTarget}"
+      # ^^ if the project is not a library, and we have a build target, then use "copy" to install
+      # just the target specified; "install" will error here, since not all targets have been built.
+    else ''
       ${setupCommand} copy
       local packageConfDir="$out/lib/${ghc.name}/package.conf.d"
       local packageConfFile="$packageConfDir/${pname}-${version}.conf"

pkgs/development/haskell-modules/lib.nix

···
   appendPatch = drv: x: appendPatches drv [x];
   appendPatches = drv: xs: overrideCabal drv (drv: { patches = (drv.patches or []) ++ xs; });
 
+  /* Set a specific build target instead of compiling all targets in the package.
+   * For example, imagine we have a .cabal file with a library, and 2 executables "dev" and "server".
+   * We can build only "server" and not wait on the compilation of "dev" by useing setBuildTarget as follows:
+   *
+   *   setBuildTarget (callCabal2nix "thePackageName" thePackageSrc {}) "server"
+   *
+   */
+  setBuildTargets = drv: xs: overrideCabal drv (drv: { buildTarget = lib.concatStringsSep " " xs; });
+  setBuildTarget = drv: x: setBuildTargets drv [x];
+
   doHyperlinkSource = drv: overrideCabal drv (drv: { hyperlinkSource = true; });
   dontHyperlinkSource = drv: overrideCabal drv (drv: { hyperlinkSource = false; });
 

pkgs/test/haskell/default.nix

···
 lib.recurseIntoAttrs {
   shellFor = callPackage ./shellFor { };
   documentationTarball = callPackage ./documentationTarball { };
+  setBuildTarget = callPackage ./setBuildTarget { };
 }

pkgs/test/haskell/setBuildTarget/Bar.hs

···
+module Main where
+
+main :: IO ()
+main = putStrLn "Hello, Bar!"

pkgs/test/haskell/setBuildTarget/Foo.hs

···
+module Main where
+
+main :: IO ()
+main = putStrLn "Hello, Foo!"

pkgs/test/haskell/setBuildTarget/Setup.hs

···
+import Distribution.Simple
+main = defaultMain

pkgs/test/haskell/setBuildTarget/default.nix

···
+{ pkgs, haskellPackages }: with pkgs.haskell.lib;
+
+
+let
+  drv     = haskellPackages.callCabal2nix "haskell-setBuildTarget" ./. {};
+  test    = target: excluded:
+
+    let only = setBuildTarget drv target;
+    in ''
+         if [[ ! -f "${only}/bin/${target}" ]]; then
+           echo "${target} was not built"
+           exit 1
+         fi
+
+         if [[ -f "${only}/bin/${excluded}" ]]; then
+           echo "${excluded} was built, when it should not have been"
+           exit 1
+         fi
+     '';
+
+in pkgs.runCommand "test haskell.lib.setBuildTarget" {} ''
+  ${test "foo" "bar"}
+  ${test "bar" "foo"}
+  touch "$out"
+''
+

pkgs/test/haskell/setBuildTarget/haskell-setBuildTarget.cabal

···
+cabal-version:       >=1.10
+name:                haskell-setBuildTarget
+version:             0.1.0.0
+author:              Isaac Shapira
+maintainer:          [e-mail]
+build-type:          Simple
+
+executable foo
+  main-is:             Foo.hs
+  build-depends:       base
+  default-language:    Haskell2010
+
+executable bar
+  main-is:             Bar.hs
+  build-depends:       base
+  default-language:    Haskell2010