Pick the latest version when the same package is in plan.json twice (input-output-hk#109)

Currently haskell.nix assums that each package will occur only once in the plan.json file.  It turns out this is not always the case.  If a package is included as a dependency of a `setup` component or `build-tool-dependencies` then it may be listed more than once.  Currently we pick the first one in the `plan.json` file (via `Map.fromList`).  This is not ideal as it is often an old version included to satisfy one of the `setup` or `build-tool-dependencies`.  In particular `cabal-doctest` often results in picking an old version of `Win32` when cross compiling for windows.

It can be difficult to get rid of the duplicates in the `plan.json` file itself (as a work around), because constraints cannot be added to the build time dependencies in `cabal.project`.

The correct fix is to add support for multiple versions of the same package in haskell.nix, but this will require a lot of refactoring.

This PR will help improve the situation by picking the newest version with `Map.fromListWith`, rather than just the first in the file.

Author: hamishmack

plan2nix/Plan2Nix.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE LambdaCase, OverloadedStrings, NamedFieldPuns, RecordWildCards #-}
+{-# LANGUAGE LambdaCase, OverloadedStrings, NamedFieldPuns, RecordWildCards, TupleSections #-}
 
 module Plan2Nix
   ( doPlan2Nix
@@ -33,6 +33,8 @@ import Data.Text.Prettyprint.Doc.Render.Text (hPutDoc)
 import Distribution.Types.PackageId (PackageIdentifier(..))
 import Distribution.Nixpkgs.Fetch (DerivationSource(..), Source(..), Hash(..), fetch)
 import Distribution.Simple.Utils (shortRelativePath)
+import Distribution.Types.Version (Version)
+import Distribution.Parsec (simpleParsec)
 
 import Control.Monad.Trans.Maybe
 import Control.Monad.IO.Class (liftIO)
@@ -241,11 +243,15 @@ value2plan plan = Plan { packages, components, extras, compilerVersion, compiler
     else Just $ pkg ^. key "pkg-version" . _String
 
   filterInstallPlan :: (Value -> Maybe b) -> HashMap Text b
-  filterInstallPlan f =
-    Map.fromList
-      $ mapMaybe (\pkg -> (,) (pkg ^. key "pkg-name" . _String) <$> f pkg)
+  filterInstallPlan f = fmap snd .
+    -- If the same package occurs more than once, choose the latest
+    Map.fromListWith (\a b -> if parseVersion (fst a) > parseVersion (fst b) then a else b)
+      $ mapMaybe (\pkg -> (,) (pkg ^. key "pkg-name" . _String) . (pkg ^. key "pkg-version" . _String,) <$> f pkg)
       $ Vector.toList (plan ^. key "install-plan" . _Array)
 
+  parseVersion :: Text -> Version
+  parseVersion s = fromMaybe (error $ "Unable to parse version " <> show s) . simpleParsec $ Text.unpack s
+
   -- Set of components that are included in the plan.
   components :: HashSet Text
   components =