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

3 files changed, 26 insertions, 1 deletions

diff --git a/libraries/base/Control/Monad.hs b/libraries/base/Control/Monad.hs
index 7de41bacc6..6957ad464f 100644
--- a/libraries/base/Control/Monad.hs
+++ b/libraries/base/Control/Monad.hs
@@ -75,8 +75,8 @@ module Control.Monad
     , (<$!>)
     ) where
 
-import Data.Functor ( void, (<$>) )
 import Data.Foldable ( Foldable, sequence_, sequenceA_, msum, mapM_, foldlM, forM_ )
+import Data.Functor ( void, (<$>) )
 import Data.Traversable ( forM, mapM, traverse, sequence, sequenceA )
 
 import GHC.Base hiding ( mapM, sequence )
diff --git a/libraries/base/Text/ParserCombinators/ReadP.hs b/libraries/base/Text/ParserCombinators/ReadP.hs
index bae2abc90e..3908b24408 100644
--- a/libraries/base/Text/ParserCombinators/ReadP.hs
+++ b/libraries/base/Text/ParserCombinators/ReadP.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE DeriveFunctor #-}
@@ -76,6 +77,10 @@ import GHC.Unicode ( isSpace )
 import GHC.List ( replicate, null )
 import GHC.Base hiding ( many )
 
+#if __GLASGOW_HASKELL__ > 710
+import Control.Monad.Fail
+#endif
+
 infixr 5 +++, <++
 
 ------------------------------------------------------------------------
@@ -119,6 +124,11 @@ instance Monad P where
 
   fail _ = Fail
 
+#if __GLASGOW_HASKELL__ > 710
+instance MonadFail P where
+  fail _ = Fail
+#endif
+
 instance Alternative P where
   empty = Fail
 
@@ -166,6 +176,11 @@ instance Monad ReadP where
   fail _    = R (\_ -> Fail)
   R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k))
 
+#if __GLASGOW_HASKELL__ > 710
+instance MonadFail ReadP where
+  fail _    = R (\_ -> Fail)
+#endif
+
 instance Alternative ReadP where
     empty = mzero
     (<|>) = mplus
diff --git a/libraries/base/Text/ParserCombinators/ReadPrec.hs b/libraries/base/Text/ParserCombinators/ReadPrec.hs
index 02268364ca..a1ce920ce2 100644
--- a/libraries/base/Text/ParserCombinators/ReadPrec.hs
+++ b/libraries/base/Text/ParserCombinators/ReadPrec.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE Trustworthy #-}
 {-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE CPP #-}
 
 -----------------------------------------------------------------------------
 -- |
@@ -64,6 +65,10 @@ import qualified Text.ParserCombinators.ReadP as ReadP
 import GHC.Num( Num(..) )
 import GHC.Base
 
+#if __GLASGOW_HASKELL__ > 710
+import qualified Control.Monad.Fail as MonadFail
+#endif
+
 -- ---------------------------------------------------------------------------
 -- The readPrec type
 
@@ -82,6 +87,11 @@ instance Monad ReadPrec where
   fail s    = P (\_ -> fail s)
   P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
 
+#if __GLASGOW_HASKELL__ > 710
+instance MonadFail.MonadFail ReadPrec where
+  fail s    = P (\_ -> fail s)
+#endif
+
 instance MonadPlus ReadPrec where
   mzero = pfail
   mplus = (+++)