This is a compatibility fix for base-4.8. All MonadPlus definitions
have Alternative as a prerequisite. Hence, instead of defining
Alternative in terms of MonadPlus, we define MonadPlus in terms of
Alternative.
Signed-off-by: Bhimanavajjula Aditya <b...@google.com>
---
src/Ganeti/BasicTypes.hs | 33 +++++++++++++++++----------------
1 file changed, 17 insertions(+), 16 deletions(-)
diff --git a/src/Ganeti/BasicTypes.hs b/src/Ganeti/BasicTypes.hs
index 340aa41..0591fa3 100644
--- a/src/Ganeti/BasicTypes.hs
+++ b/src/Ganeti/BasicTypes.hs
@@ -128,13 +128,17 @@ instance Functor (GenericResult a) where
fmap _ (Bad msg) = Bad msg
fmap fn (Ok val) = Ok (fn val)
-instance (Error a, Monoid a) => MonadPlus (GenericResult a) where
- mzero = Bad $ strMsg "zero Result when used as MonadPlus"
+instance (Error a, Monoid a) => Alternative (GenericResult a) where
+ empty = Bad $ strMsg "zero Result when used as empty"
-- for mplus, when we 'add' two Bad values, we concatenate their
-- error descriptions
- (Bad x) `mplus` (Bad y) = Bad (x `mappend` strMsg "; " `mappend` y)
- (Bad _) `mplus` x = x
- x@(Ok _) `mplus` _ = x
+ (Bad x) <|> (Bad y) = Bad (x `mappend` strMsg "; " `mappend` y)
+ (Bad _) <|> x = x
+ x@(Ok _) <|> _ = x
+
+instance (Error a, Monoid a) => MonadPlus (GenericResult a) where
+ mzero = empty
+ mplus = (<|>)
instance (Error a) => MonadError a (GenericResult a) where
throwError = Bad
@@ -148,10 +152,6 @@ instance Applicative (GenericResult a) where
_ <*> (Bad x) = Bad x
(Ok f) <*> (Ok x) = Ok $ f x
-instance (Error a, Monoid a) => Alternative (GenericResult a) where
- empty = mzero
- (<|>) = mplus
-
-- | This is a monad transformation for Result. It's implementation is
-- based on the implementations of MaybeT and ErrorT.
--
@@ -238,17 +238,18 @@ instance (Error a, MonadBaseControl IO m)
{-# INLINE liftBaseWith #-}
{-# INLINE restoreM #-}
-instance (Monad m, Error a, Monoid a) => MonadPlus (ResultT a m) where
- mzero = ResultT $ return mzero
+instance (Monad m, Error a, Monoid a)
+ => Alternative (ResultT a m) where
+ empty = ResultT $ return mzero
-- Ensure that 'y' isn't run if 'x' contains a value. This makes it a bit
-- more complicated than 'mplus' of 'GenericResult'.
- mplus x y = elimResultT combine return x
+ x <|> y = elimResultT combine return x
where combine x' = ResultT $ liftM (mplus (Bad x')) (runResultT y)
-instance (Alternative m, Monad m, Error a, Monoid a)
- => Alternative (ResultT a m) where
- empty = mzero
- (<|>) = mplus
+instance (Monad m, Error a, Monoid a)
+ => MonadPlus (ResultT a m) where
+ mzero = empty
+ mplus = (<|>)
-- | Changes the error message of a result value, if present.
-- Note that since 'GenericResult' is also a 'MonadError', this function
--
2.5.0.457.gab17608
