module Control.Monad.Fix (
MonadFix(
mfix
),
fix
) where
import Prelude
import System.IO
import Control.Monad.Instances ()
import Data.Function (fix)
#ifdef __HUGS__
import Hugs.Prelude (MonadFix(mfix))
#endif
#if defined(__GLASGOW_HASKELL__)
import GHC.ST
#endif
#ifndef __HUGS__
class (Monad m) => MonadFix m where
mfix :: (a -> m a) -> m a
#endif /* !__HUGS__ */
instance MonadFix Maybe where
mfix f = let a = f (unJust a) in a
where unJust (Just x) = x
unJust Nothing = error "mfix Maybe: Nothing"
instance MonadFix [] where
mfix f = case fix (f . head) of
[] -> []
(x:_) -> x : mfix (tail . f)
instance MonadFix IO where
mfix = fixIO
instance MonadFix ((->) r) where
mfix f = \ r -> let a = f a r in a
instance MonadFix (Either e) where
mfix f = let a = f (unRight a) in a
where unRight (Right x) = x
unRight (Left _) = error "mfix Either: Left"
#if defined(__GLASGOW_HASKELL__)
instance MonadFix (ST s) where
mfix = fixST
#endif
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