MonadFail proposal, phase 1

This implements phase 1 of the MonadFail proposal (MFP, #10751).

- MonadFail warnings are all issued as desired, tunable with two new flags
- GHC was *not* made warning-free with `-fwarn-missing-monadfail-warnings`
  (but it's disabled by default right now)

Credits/thanks to
- Franz Thoma, whose help was crucial to implementing this
- My employer TNG Technology Consulting GmbH for partially funding us
  for this work

Reviewers: goldfire, austin, #core_libraries_committee, hvr, bgamari, fmthoma

Reviewed By: hvr, bgamari, fmthoma

Subscribers: thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D1248

GHC Trac Issues: #10751

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/utils/Maybes.hs b/compiler/utils/Maybes.hs
index 56b6dab5d9..656f40a372 100644
--- a/compiler/utils/Maybes.hs
+++ b/compiler/utils/Maybes.hs
@@ -20,6 +20,9 @@ module Maybes (
 
 import Control.Applicative
 import Control.Monad
+#if __GLASGOW_HASKELL__ > 710
+import Control.Monad.Fail
+#endif
 import Data.Maybe
 
 infixr 4 `orElse`
@@ -85,6 +88,12 @@ instance (Monad m) => Monad (MaybeT m) where
   x >>= f = MaybeT $ runMaybeT x >>= maybe (pure Nothing) (runMaybeT . f)
   fail _ = MaybeT $ pure Nothing
 
+
+#if __GLASGOW_HASKELL__ > 710
+instance Monad m => MonadFail (MaybeT m) where
+  fail _ = MaybeT $ return Nothing
+#endif
+
 #if __GLASGOW_HASKELL__ < 710
 -- Pre-AMP change
 instance (Monad m, Applicative m) => Alternative (MaybeT m) where