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|
{-# LANGUAGE NoImplicitPrelude #-}
module Control.Monad.Fix (
MonadFix(mfix),
fix
) where
import Control.Monad
import Data.Function (fix,(.))
import Data.Maybe
import Compiler.Error (error)
import Data.OldList (head, tail)
import Compiler.IO
import Compiler.ST
--Laws:
--
-- Purity
-- mfix (return . h) = return (fix h)
-- Left shrinking (or Tightening)
-- mfix (\x -> a >>= \y -> f x y) = a >>= \y -> mfix (\x -> f x y)
-- Sliding
-- mfix (liftM h . f) = liftM h (mfix (f . h)), for strict h.
-- Nesting
-- mfix (\x -> mfix (\y -> f x y)) = mfix (\x -> f x x)
class Monad m => MonadFix m where
mfix :: (a -> m a) -> m a
instance MonadFix [] where
mfix f = case fix (f . head) of
[] -> []
(x:_) -> x : mfix (tail . f)
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 IO where
mfix = fixIO
instance MonadFix (ST s) where
mfix = fixST
|
