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diff --git a/src/Control/Monad/LogicState.hs b/src/Control/Monad/LogicState.hs
index 93be8aa..613a77c 100644
--- a/src/Control/Monad/LogicState.hs
+++ b/src/Control/Monad/LogicState.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE UndecidableInstances, Rank2Types, FlexibleInstances, FlexibleContexts, GADTs, ScopedTypeVariables, FunctionalDependencies #-}
+{-# LANGUAGE UndecidableInstances, Rank2Types, FlexibleInstances, FlexibleContexts, GADTs, ScopedTypeVariables, FunctionalDependencies, CPP #-}
-------------------------------------------------------------------------
-- |
@@ -39,6 +39,9 @@ import Control.Applicative
import Control.Monad
import Control.Monad.Identity
+#if !MIN_VERSION_base(4,11,0)
+import qualified Control.Monad.Fail as Fail
+#endif
import Control.Monad.Trans
import Control.Monad.State
@@ -83,7 +86,12 @@ instance Applicative (LogicStateT gs bs f) where
instance Monad (LogicStateT gs bs m) where
return a = LogicStateT ($ a)
m >>= f = LogicStateT $ \sk -> unLogicStateT m (\a -> unLogicStateT (f a) sk)
- fail _ = LogicStateT $ flip const
+#if !MIN_VERSION_base(4,11,0)
+ fail = Fail.fail
+#endif
+
+instance MonadFail (LogicStateT gs bs m) where
+ fail _ = LogicStateT $ flip const
instance Alternative (LogicStateT gs bs f) where
empty = LogicStateT $ flip const
diff --git a/src/Control/Monad/TransLogicState/Class.hs b/src/Control/Monad/TransLogicState/Class.hs
index 4fa61c4..267704a 100644
--- a/src/Control/Monad/TransLogicState/Class.hs
+++ b/src/Control/Monad/TransLogicState/Class.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE ScopedTypeVariables, CPP #-}
module Control.Monad.TransLogicState.Class
( TransLogicState(..)
@@ -15,12 +15,19 @@ import Control.Arrow
import Control.Monad.Identity
-- import Control.Monad.Trans
+instance MonadFail Identity where
+ fail msg = runIdentity $ fail msg
+
-- | Additions to MonadTrans specifically useful for LogicState
class {- MonadTrans t => -} TransLogicState s t where
-------------------------------------------------------------------------
-- | Extracts the first result from a 't m' computation,
-- failing otherwise.
+#if !MIN_VERSION_base(4,13,0)
observeT :: (Monad m) => s -> t m a -> m a
+#else
+ observeT :: (MonadFail m) => s -> t m a -> m a
+#endif
observeT e m = fmap head $ observeManyT e 1 m
-------------------------------------------------------------------------
|
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