1 -----------------------------------------------------------------------------
3 -- Module : Control.Monad.Fix
4 -- Copyright : (c) Andy Gill 2001,
5 -- (c) Oregon Graduate Institute of Science and Technology, 2002
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : portable
13 -- For more details, see Levent Erkok's thesis,
14 -- /Value Recursion in Monadic Computations/, 2002.
16 -----------------------------------------------------------------------------
18 module Control.Monad.Fix (
20 mfix -- :: (a -> m a) -> m a
22 fix -- :: (a -> a) -> a
28 -- | @'fix' f@ is the least fixed point of the function @f@.
30 fix f = let x = f x in x
32 -- | Monads having fixed points with a \`knot-tying\' semantics.
33 -- Instances of 'MonadFix' should satisfy the laws
36 -- @'mfix' ('return' . h) = 'return' ('fix' h)@
38 -- [left shrinking (or tightening)]
39 -- @'mfix' (\x -> a >>= \y -> f x y) = \y -> 'mfix' (\x -> f x y)@
42 -- @'mfix' ('Control.Monad.liftM' h . f) = 'Control.Monad.liftM' h ('mfix' (f . h))@,
46 -- @'mfix' (\x -> 'mfix' (\y -> f x y)) = 'mfix' (\x -> f x x)@
48 -- This class is used in the translation of the recursive @do@ notation
49 -- supported by GHC and Hugs.
50 class (Monad m) => MonadFix m where
51 -- | The fixed point of a monadic computation.
52 -- @'mfix' f@ executes the action 'f' only once, with the eventual
53 -- output fed back as the input. Hence @f@ should not be strict,
54 -- for then @'mfix' f@ would diverge.
55 mfix :: (a -> m a) -> m a
57 -- Instances of MonadFix for Prelude monads
60 instance MonadFix Maybe where
61 mfix f = let a = f (unJust a) in a
62 where unJust (Just x) = x
65 instance MonadFix [] where
66 mfix f = case fix (f . head) of
68 (x:_) -> x : mfix (tail . f)
71 instance MonadFix IO where