Refactoring to use cabal-helper-wrapper

This turned out to be quite involved but save for this huge commit it's
actually quite awesome and squashes quite a few bugs and nasty
problems (hopefully). Most importantly we now have native cabal
component support without the user having to do anything to get it!

To do this we traverse imports starting from each component's
entrypoints (library modules or Main source file for executables) and
use this information to find which component's options each module will
build with. Under the assumption that these modules have to build with
every component they're used in we can now just pick one.

Quite a few internal assumptions have been invalidated by this
change. Most importantly the runGhcModT* family of cuntions now change
the current working directory to `cradleRootDir`.

Language/Haskell/GhcMod/Cradle.hs

@@ -1,19 +1,21 @@
 module Language.Haskell.GhcMod.Cradle (
     findCradle
   , findCradle'
-  , findCradleWithoutSandbox
+  , findSpecCradle
   , cleanupCradle
   ) where
 
-import Language.Haskell.GhcMod.GhcPkg
 import Language.Haskell.GhcMod.PathsAndFiles
+import Language.Haskell.GhcMod.Monad.Types
 import Language.Haskell.GhcMod.Types
 import Language.Haskell.GhcMod.Utils
 
-import Control.Exception.IOChoice ((||>))
-import System.Directory (getCurrentDirectory, removeDirectoryRecursive)
-import System.FilePath (takeDirectory)
-
+import Control.Applicative
+import Control.Monad
+import Control.Monad.Trans.Maybe
+import Data.Maybe
+import System.Directory
+import System.FilePath
 
 ----------------------------------------------------------------
 
@@ -25,51 +27,71 @@ findCradle :: IO Cradle
 findCradle = findCradle' =<< getCurrentDirectory
 
 findCradle' :: FilePath -> IO Cradle
-findCradle' dir = cabalCradle dir ||> sandboxCradle dir ||> plainCradle dir
+findCradle' dir = run $ do
+    (cabalCradle dir `mplus`  sandboxCradle dir `mplus` plainCradle dir)
+ where run a = fillTempDir =<< (fromJust <$> runMaybeT a)
+
+findSpecCradle :: FilePath -> IO Cradle
+findSpecCradle dir = do
+    let cfs = [cabalCradle, sandboxCradle]
+    cs <- catMaybes <$> mapM (runMaybeT . ($ dir)) cfs
+    gcs <- filterM isNotGmCradle cs
+    fillTempDir =<< case gcs of
+                      [] -> fromJust <$> runMaybeT (plainCradle dir)
+                      c:_ -> return c
+ where
+   isNotGmCradle :: Cradle -> IO Bool
+   isNotGmCradle crdl = do
+     not <$> doesFileExist (cradleRootDir crdl </> "ghc-mod.cabal")
 
 cleanupCradle :: Cradle -> IO ()
 cleanupCradle crdl = removeDirectoryRecursive $ cradleTempDir crdl
 
-cabalCradle :: FilePath -> IO Cradle
+fillTempDir :: MonadIO m => Cradle -> m Cradle
+fillTempDir crdl = do
+  tmpDir <- liftIO $ newTempDir (cradleRootDir crdl)
+  return crdl { cradleTempDir = tmpDir }
+
+cabalCradle :: FilePath -> MaybeT IO Cradle
 cabalCradle wdir = do
-    Just cabalFile <- findCabalFile wdir
+    cabalFile <- MaybeT $ findCabalFile wdir
+
     let cabalDir = takeDirectory cabalFile
-    pkgDbStack <- getPackageDbStack cabalDir
-    tmpDir <- newTempDir cabalDir
+    pkgDbStack <- liftIO $ getPackageDbStack cabalDir
+
     return Cradle {
         cradleCurrentDir = wdir
       , cradleRootDir    = cabalDir
-      , cradleTempDir    = tmpDir
+      , cradleTempDir    = error "tmpDir"
       , cradleCabalFile  = Just cabalFile
       , cradlePkgDbStack = pkgDbStack
       }
 
-sandboxCradle :: FilePath -> IO Cradle
+sandboxCradle :: FilePath -> MaybeT IO Cradle
 sandboxCradle wdir = do
-    Just sbDir <- findCabalSandboxDir wdir
-    pkgDbStack <- getPackageDbStack sbDir
-    tmpDir <- newTempDir sbDir
+    sbDir <- MaybeT $ findCabalSandboxDir wdir
+    pkgDbStack <- liftIO $ getPackageDbStack sbDir
     return Cradle {
         cradleCurrentDir = wdir
       , cradleRootDir    = sbDir
-      , cradleTempDir    = tmpDir
+      , cradleTempDir    = error "tmpDir"
       , cradleCabalFile  = Nothing
       , cradlePkgDbStack = pkgDbStack
       }
 
-plainCradle :: FilePath -> IO Cradle
+plainCradle :: FilePath -> MaybeT IO Cradle
 plainCradle wdir = do
-    tmpDir <- newTempDir wdir
-    return Cradle {
+    return $ Cradle {
         cradleCurrentDir = wdir
       , cradleRootDir    = wdir
-      , cradleTempDir    = tmpDir
+      , cradleTempDir    = error "tmpDir"
       , cradleCabalFile  = Nothing
       , cradlePkgDbStack = [GlobalDb, UserDb]
       }
 
--- Just for testing
-findCradleWithoutSandbox :: IO Cradle
-findCradleWithoutSandbox = do
-    cradle <- findCradle
-    return cradle { cradlePkgDbStack = [GlobalDb]} -- FIXME
+getPackageDbStack :: FilePath -- ^ Project Directory (where the
+                                 -- cabal.sandbox.config file would be if it
+                                 -- exists)
+                  -> IO [GhcPkgDb]
+getPackageDbStack cdir =
+    ([GlobalDb] ++) . maybe [UserDb] return <$> getSandboxDb cdir