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, 8 insertions, 0 deletions

diff --git a/compiler/types/Unify.hs b/compiler/types/Unify.hs
index 87681e0eb8..a29c85f2da 100644
--- a/compiler/types/Unify.hs
+++ b/compiler/types/Unify.hs
@@ -34,6 +34,9 @@ import Outputable
 import FastString (sLit)
 
 import Control.Monad (liftM, foldM, ap)
+#if __GLASGOW_HASKELL__ > 710
+import qualified Control.Monad.Fail as MonadFail
+#endif
 #if __GLASGOW_HASKELL__ < 709
 import Control.Applicative (Applicative(..))
 #endif
@@ -729,6 +732,11 @@ instance Monad UM where
                                other                   -> other
                            SurelyApart -> SurelyApart)
 
+#if __GLASGOW_HASKELL__ > 710
+instance MonadFail.MonadFail UM where
+    fail _   = UM (\_tvs _subst -> SurelyApart) -- failed pattern match
+#endif
+
 -- returns an idempotent substitution
 initUM :: (TyVar -> BindFlag) -> UM () -> UnifyResult
 initUM badtvs um = fmap (niFixTvSubst . snd) $ unUM um badtvs emptyTvSubstEnv