haskell: add the checkUnusedPackages function including docs

doc/languages-frameworks/haskell.md

@@ -867,6 +867,62 @@ use the following to get the `scientific` package build with `integer-simple`:
 nix-build -A haskell.packages.integer-simple.ghc802.scientific
 ```
 
+### Quality assurance
+
+The `haskell.lib` library includes a number of functions for checking for
+various imperfections in Haskell packages. It's useful to apply these functions
+to your own Haskell packages and integrate that in a Continuous Integration
+server like [hydra](https://nixos.org/hydra/) to assure your packages maintain a
+minimum level of quality. This section discusses some of these functions.
+
+#### buildStrictly
+
+Applying `haskell.lib.buildStrictly` to a Haskell package enables the `-Wall`
+and `-Werror` GHC options to turn all warnings into build failures. Additionally
+the source of your package is gotten from first invoking `cabal sdist` to ensure
+all needed files are listed in the Cabal file.
+
+#### checkUnusedPackages
+
+Applying `haskell.lib.checkUnusedPackages` to a Haskell package invokes
+the [packunused](http://hackage.haskell.org/package/packunused) tool on the
+package. `packunused` complains when it finds packages listed as build-depends
+in the Cabal file which are redundant. For example:
+
+```
+$ nix-build -E 'let pkgs = import <nixpkgs> {}; in pkgs.haskell.lib.checkUnusedPackages {} pkgs.haskellPackages.scientific'
+these derivations will be built:
+  /nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv
+...
+detected package components
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ - library
+ - testsuite(s): test-scientific
+ - benchmark(s): bench-scientific*
+
+(component names suffixed with '*' are not configured to be built)
+
+library
+~~~~~~~
+
+The following package dependencies seem redundant:
+
+ - ghc-prim-0.5.0.0
+
+testsuite(test-scientific)
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+no redundant packages dependencies found
+
+builder for ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv’ failed with exit code 1
+error: build of ‘/nix/store/3lc51cxj2j57y3zfpq5i69qbzjpvyci1-scientific-0.3.5.1.drv’ failed
+```
+
+As you can see, `packunused` finds out that although the testsuite component has
+no redundant dependencies the library component of `scientific-0.3.5.1` depends
+on `ghc-prim` which is unused in the library.
+
 ## Other resources
 
   - The Youtube video [Nix Loves Haskell](https://www.youtube.com/watch?v=BsBhi_r-OeE)

pkgs/development/haskell-modules/lib.nix

@@ -1,5 +1,5 @@
 # TODO(@Ericson2314): Remove `pkgs` param, which is only used for
-# `buildStackProject` and `justStaticExecutables`
+# `buildStackProject`, `justStaticExecutables` and `checkUnusedPackages`
 { pkgs, lib }:
 
 rec {
@@ -108,6 +108,22 @@ rec {
 
   buildStrictly = pkg: buildFromSdist (appendConfigureFlag pkg "--ghc-option=-Wall --ghc-option=-Werror");
 
+  checkUnusedPackages =
+    { ignoreEmptyImports ? false
+    , ignoreMainModule   ? false
+    , ignorePackages     ? []
+    } : drv :
+      overrideCabal (appendConfigureFlag drv "--ghc-option=-ddump-minimal-imports") (_drv: {
+        postBuild = with lib;
+          let args = concatStringsSep " " (
+                       optional ignoreEmptyImports "--ignore-empty-imports" ++
+                       optional ignoreMainModule   "--ignore-main-module" ++
+                       map (pkg: "--ignore-package ${pkg}") ignorePackages
+                     );
+          in "${pkgs.haskellPackages.packunused}/bin/packunused" +
+             optionalString (args != "") " ${args}";
+      });
+
   buildStackProject = pkgs.callPackage ./generic-stack-builder.nix { };
 
   triggerRebuild = drv: i: overrideCabal drv (drv: { postUnpack = ": trigger rebuild ${toString i}"; });