Add missing MonadFail instances (when base >=4.9) for ConcT/STM

Closes #136

dejafu/CHANGELOG.markdown

@@ -14,6 +14,14 @@ unreleased
 
 - Fix some incorrect "@since" haddock comments.
 
+### Test.DejaFu.Conc
+
+- Add a missing `MonadFail` instance.
+
+### Test.DejaFu.STM
+
+- Add a missing `MonadFail` instance.
+
 
 ---------------------------------------------------------------------------------------------------
 

dejafu/Test/DejaFu/Conc.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
@@ -11,7 +12,7 @@
 -- License     : MIT
 -- Maintainer  : Michael Walker <mike@barrucadu.co.uk>
 -- Stability   : experimental
--- Portability : FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, RankNTypes, TypeFamilies, TypeSynonymInstances
+-- Portability : CPP, FlexibleInstances, GeneralizedNewtypeDeriving, MultiParamTypeClasses, RankNTypes, TypeFamilies, TypeSynonymInstances
 --
 -- Deterministic traced execution of concurrent computations.
 --
@@ -65,9 +66,19 @@ import           Test.DejaFu.Conc.Internal
 import           Test.DejaFu.Conc.Internal.Common
 import           Test.DejaFu.STM
 
+#if MIN_VERSION_base(4,9,0)
+import qualified Control.Monad.Fail               as Fail
+#endif
+
 -- | @since 0.6.0.0
 newtype ConcT r n a = C { unC :: M n r a } deriving (Functor, Applicative, Monad)
 
+#if MIN_VERSION_base(4,9,0)
+-- | @since unreleased
+instance Fail.MonadFail (ConcT r n) where
+  fail = C . fail
+#endif
+
 -- | A 'MonadConc' implementation using @ST@, this should be preferred
 -- if you do not need 'liftIO'.
 --

dejafu/Test/DejaFu/STM.hs

@@ -39,9 +39,19 @@ import qualified Control.Monad.STM.Class  as C
 import           Test.DejaFu.Common
 import           Test.DejaFu.STM.Internal
 
+#if MIN_VERSION_base(4,9,0)
+import qualified Control.Monad.Fail       as Fail
+#endif
+
 -- | @since 0.3.0.0
 newtype STMLike n r a = S { runSTM :: M n r a } deriving (Functor, Applicative, Monad)
 
+#if MIN_VERSION_base(4,9,0)
+-- | @since unreleased
+instance Fail.MonadFail (STMLike r n) where
+  fail = S . fail
+#endif
+
 -- | Create a new STM continuation.
 toSTM :: ((a -> STMAction n r) -> STMAction n r) -> STMLike n r a
 toSTM = S . cont