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

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

@@ -464,7 +464,12 @@ stdenv.mkDerivation ({
   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

@@ -196,6 +196,16 @@ rec {
   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; });