Added withMappedFile utility function

Works pretty much as `withTempFile`, except looks
mapping up and uses temp. file only when necessary.

Language/Haskell/GhcMod/FileMappingUtils.hs

@@ -0,0 +1,22 @@
+module Language.Haskell.GhcMod.FileMappingUtils where
+
+import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.Monad.Types
+
+import System.IO
+import System.FilePath
+import System.Directory
+
+withMappedFile :: (IOish m, GmState m, GmEnv m) => forall a. FilePath -> (FilePath -> m a) -> m a
+withMappedFile file action = lookupMMappedFile file >>= runWithFile
+  where
+    runWithFile (Just (RedirectedMapping to)) = action to
+    runWithFile (Just (MemoryMapping (Just src))) = do
+      crdl <- cradle
+      (fp,hndl) <- liftIO $ openTempFile (cradleTempDir crdl) (takeBaseName file)
+      liftIO $ hPutStr hndl src
+      liftIO $ hClose hndl
+      result <- action fp
+      liftIO $ removeFile fp
+      return result
+    runWithFile _ = action file

Language/Haskell/GhcMod/HomeModuleGraph.hs

@@ -61,8 +61,7 @@ import Language.Haskell.GhcMod.Logger
 import Language.Haskell.GhcMod.Monad.Types
 import Language.Haskell.GhcMod.Types
 import Language.Haskell.GhcMod.Gap (parseModuleHeader)
-
-import System.IO
+import Language.Haskell.GhcMod.FileMappingUtils
 
 -- | Turn module graph into a graphviz dot file
 --

ghc-mod.cabal

@@ -106,6 +106,7 @@ Library
                         Language.Haskell.GhcMod.DynFlags
                         Language.Haskell.GhcMod.Error
                         Language.Haskell.GhcMod.FileMapping
+                        Language.Haskell.GhcMod.FileMappingUtils
                         Language.Haskell.GhcMod.FillSig
                         Language.Haskell.GhcMod.Find
                         Language.Haskell.GhcMod.Flag