From 4f785d7f8a82c29f493851cab9e2633d80425e3c Mon Sep 17 00:00:00 2001
From: Kazu Yamamoto <kazu@iij.ad.jp>
Date: Thu, 14 Aug 2014 14:13:09 +0900
Subject: [PATCH] Set Opt_DeferTypeErrors to rescure type errors. (#310)

Type errors are turned to warnings. We force to turn them to the errors again.
---
 Language/Haskell/GhcMod/Check.hs  | 2 +-
 Language/Haskell/GhcMod/Logger.hs | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/Language/Haskell/GhcMod/Check.hs b/Language/Haskell/GhcMod/Check.hs
index 6d30f55..ea2e28a 100644
--- a/Language/Haskell/GhcMod/Check.hs
+++ b/Language/Haskell/GhcMod/Check.hs
@@ -37,7 +37,7 @@ check :: IOish m
       => [FilePath]  -- ^ The target files.
       -> GhcModT m (Either String String)
 check fileNames = overrideGhcUserOptions $ \ghcOpts ->
-  withLogger (setAllWaringFlags . setNoMaxRelevantBindings) $ do
+  withLogger (setAllWaringFlags . setNoMaxRelevantBindings . Gap.setWarnTypedHoles . Gap.setDeferTypeErrors) $ do
     _ <- G.setSessionDynFlags =<< addCmdOpts ghcOpts =<< G.getSessionDynFlags
     setTargetFiles fileNames
 
diff --git a/Language/Haskell/GhcMod/Logger.hs b/Language/Haskell/GhcMod/Logger.hs
index 0263561..d3b712b 100644
--- a/Language/Haskell/GhcMod/Logger.hs
+++ b/Language/Haskell/GhcMod/Logger.hs
@@ -105,7 +105,13 @@ ppMsg spn sev dflag style msg = prefix ++ cts
         (line,col,_,_) <- Gap.getSrcSpan spn
         file <- normalise <$> Gap.getSrcFile spn
         let severityCaption = Gap.showSeverityCaption sev
-        return $ file ++ ":" ++ show line ++ ":" ++ show col ++ ":" ++ severityCaption
+            pref0
+              | typeWarning `isPrefixOf` cts = file ++ ":" ++ show line ++ ":" ++ show col ++ ":"
+              | otherwise                    = file ++ ":" ++ show line ++ ":" ++ show col ++ ":" ++ severityCaption
+        return pref0
+    -- DeferTypeErrors turns a type error to a warning.
+    -- So, let's turns it the error again.
+    typeWarning = "Couldn't match expected type"
 
 checkErrorPrefix :: String
 checkErrorPrefix = "Dummy:0:0:Error:"