Don't swallow ghc's verbose output

Language/Haskell/GhcMod/DynFlags.hs

@@ -3,18 +3,19 @@
 module Language.Haskell.GhcMod.DynFlags where
 
 import Control.Applicative
-import Control.Monad (void)
-import GHC (DynFlags(..), GhcMode(..), GhcLink(..), HscTarget(..))
+import Control.Monad
+import GHC
 import qualified GHC as G
 import GHC.Paths (libdir)
-import GhcMonad
 import qualified Language.Haskell.GhcMod.Gap as Gap
 import Language.Haskell.GhcMod.Types
+import Language.Haskell.GhcMod.DebugLogger
 import System.IO.Unsafe (unsafePerformIO)
 import Prelude
 
-setEmptyLogger :: DynFlags -> DynFlags
-setEmptyLogger df = Gap.setLogAction df $ \_ _ _ _ _ -> return ()
+setDebugLogger :: (String -> IO ()) -> DynFlags -> DynFlags
+setDebugLogger put df = do
+  Gap.setLogAction df (debugLogAction put)
 
 -- * Fast
 -- * Friendly to foreign export

Language/Haskell/GhcMod/Output.hs

@@ -22,9 +22,15 @@ module Language.Haskell.GhcMod.Output (
   , gmErrStr
   , gmPutStrLn
   , gmErrStrLn
+
+  , gmPutStrIO
+  , gmErrStrIO
+
   , gmReadProcess
+
   , gmUnsafePutStr
   , gmUnsafeErrStr
+
   , stdoutGateway
   ) where
 
@@ -105,15 +111,21 @@ gmPutStr, gmPutStrLn, gmErrStr, gmErrStrLn
     :: (MonadIO m, GmOut m) => String -> m ()
 
 gmPutStr str = do
-  putOut <- fst `liftM` outputFns
-  putOut $ toGmLines str
+  putOut <- gmPutStrIO
+  putOut str
+
+gmErrStr str = do
+  putErr <- gmErrStrIO
+  putErr str
 
 gmPutStrLn = gmPutStr . (++"\n")
 gmErrStrLn = gmErrStr . (++"\n")
 
-gmErrStr str = do
-  putErr <- snd `liftM` outputFns
-  putErr $ toGmLines str
+gmPutStrIO, gmErrStrIO :: (GmOut m, MonadIO mi) => m (String -> mi ())
+
+gmPutStrIO = ((. toGmLines) . fst) `liftM` outputFns
+gmErrStrIO = ((. toGmLines) . snd) `liftM` outputFns
+
 
 -- | Only use these when you're sure there are no other writers on stdout
 gmUnsafePutStr, gmUnsafeErrStr

Language/Haskell/GhcMod/Target.hs

@@ -38,6 +38,7 @@ import Language.Haskell.GhcMod.Utils as U
 import Language.Haskell.GhcMod.FileMapping
 import Language.Haskell.GhcMod.LightGhc
 import Language.Haskell.GhcMod.CustomPackageDb
+import Language.Haskell.GhcMod.Output
 
 import Data.Maybe
 import Data.Monoid as Monoid
@@ -131,8 +132,9 @@ runGmlTWith efnmns' mdf wrapper action = do
       (text "Initializing GHC session with following options")
       (intercalate " " $ map (("\""++) . (++"\"")) opts')
 
+    putErr <- gmErrStrIO
     initSession opts' $
-        setModeSimple >>> setEmptyLogger >>> mdf
+        setModeSimple >>> setDebugLogger putErr >>> mdf
 
     mappedStrs <- getMMappedFilePaths
     let targetStrs = mappedStrs ++ map moduleNameString mns ++ cfns

ghc-mod.cabal

@@ -112,6 +112,7 @@ Library
                         Language.Haskell.GhcMod.Cradle
                         Language.Haskell.GhcMod.CustomPackageDb
                         Language.Haskell.GhcMod.Debug
+                        Language.Haskell.GhcMod.DebugLogger
                         Language.Haskell.GhcMod.Doc
                         Language.Haskell.GhcMod.DynFlags
                         Language.Haskell.GhcMod.Error