MonadFail: Make backwards-compatible

3 files changed, 19 insertions, 4 deletions

diff --git a/src/compiler/GF/Data/BacktrackM.hs b/src/compiler/GF/Data/BacktrackM.hs
index 87ea4dc5a..f947b838e 100644
--- a/src/compiler/GF/Data/BacktrackM.hs
+++ b/src/compiler/GF/Data/BacktrackM.hs
@@ -13,6 +13,7 @@
 -----------------------------------------------------------------------------
 
 {-# LANGUAGE Rank2Types, MultiParamTypeClasses, FlexibleInstances #-}
+{-# LANGUAGE CPP #-}
 module GF.Data.BacktrackM (
                     -- * the backtracking state monad
 		    BacktrackM,
@@ -32,6 +33,7 @@ import Data.List
 import Control.Applicative
 import Control.Monad
 import Control.Monad.State.Class
+import qualified Control.Monad.Fail as Fail
 
 ----------------------------------------------------------------------
 -- Combining endomorphisms and continuations
@@ -70,7 +72,12 @@ instance Monad (BacktrackM s) where
     BM m >>= k = BM (\c s b -> m (\a s b -> unBM (k a) c s b) s b)
 	where unBM (BM m) = m
 
-instance MonadFail (BacktrackM s) where
+#if !(MIN_VERSION_base(4,13,0))
+  -- Monad(fail) will be removed in GHC 8.8+
+  fail = Fail.fail
+#endif
+
+instance Fail.MonadFail (BacktrackM s) where
     fail _ = mzero
 
 instance Functor (BacktrackM s) where
diff --git a/src/compiler/GF/Data/ErrM.hs b/src/compiler/GF/Data/ErrM.hs
index 5ea7f0734..288c61919 100644
--- a/src/compiler/GF/Data/ErrM.hs
+++ b/src/compiler/GF/Data/ErrM.hs
@@ -12,10 +12,12 @@
 -- hack for BNFC generated files. AR 21/9/2003
 -----------------------------------------------------------------------------
 
+{-# LANGUAGE CPP #-}
 module GF.Data.ErrM where
 
 import Control.Monad (MonadPlus(..),ap)
 import Control.Applicative
+import qualified Control.Monad.Fail as Fail
 
 -- | Like 'Maybe' type with error msgs
 data Err a = Ok a | Bad String
@@ -36,7 +38,12 @@ instance Monad Err where
   Ok a  >>= f = f a
   Bad s >>= f = Bad s
 
-instance MonadFail Err where
+#if !(MIN_VERSION_base(4,13,0))
+  -- Monad(fail) will be removed in GHC 8.8+
+  fail = Fail.fail
+#endif
+
+instance Fail.MonadFail Err where
   fail        = Bad
 
 
diff --git a/src/compiler/GF/Data/Operations.hs b/src/compiler/GF/Data/Operations.hs
index a9fedcaff..08fa15c3e 100644
--- a/src/compiler/GF/Data/Operations.hs
+++ b/src/compiler/GF/Data/Operations.hs
@@ -53,6 +53,7 @@ import Control.Monad (liftM,liftM2) --,ap
 
 import GF.Data.ErrM
 import GF.Data.Relation
+import qualified Control.Monad.Fail as Fail
 
 infixr 5 +++
 infixr 5 ++-
@@ -88,10 +89,10 @@ checkUnique ss = ["overloaded" +++ show s | s <- nub overloads] where
   overloaded s = length (filter (==s) ss) > 1
 
 -- | this is what happens when matching two values in the same module
-unifyMaybe :: (Eq a, MonadFail m) => Maybe a -> Maybe a -> m (Maybe a)
+unifyMaybe :: (Eq a, Fail.MonadFail m) => Maybe a -> Maybe a -> m (Maybe a)
 unifyMaybe = unifyMaybeBy id
 
-unifyMaybeBy :: (Eq b, MonadFail m) => (a->b) -> Maybe a -> Maybe a -> m (Maybe a)
+unifyMaybeBy :: (Eq b, Fail.MonadFail m) => (a->b) -> Maybe a -> Maybe a -> m (Maybe a)
 unifyMaybeBy f (Just p1) (Just p2)
   | f p1==f p2                     = return (Just p1)
   | otherwise                      = fail ""