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 a detailed discussion, see Levent Erkok's thesis,
14 -- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002.
16 -----------------------------------------------------------------------------
18 module Control.Monad.Fix (
20 mfix -- :: (a -> m a) -> m a
22 fix -- :: (a -> a) -> a
27 import Control.Monad.Instances ()
28 import Data.Function (fix)
30 import Hugs.Prelude (MonadFix(mfix))
32 #if defined(__GLASGOW_HASKELL__)
37 -- | Monads having fixed points with a \'knot-tying\' semantics.
38 -- Instances of 'MonadFix' should satisfy the following laws:
41 -- @'mfix' ('return' . h) = 'return' ('fix' h)@
43 -- [/left shrinking/ (or /tightening/)]
44 -- @'mfix' (\\x -> a >>= \\y -> f x y) = a >>= \\y -> 'mfix' (\\x -> f x y)@
47 -- @'mfix' ('Control.Monad.liftM' h . f) = 'Control.Monad.liftM' h ('mfix' (f . h))@,
51 -- @'mfix' (\\x -> 'mfix' (\\y -> f x y)) = 'mfix' (\\x -> f x x)@
53 -- This class is used in the translation of the recursive @do@ notation
54 -- supported by GHC and Hugs.
55 class (Monad m) => MonadFix m where
56 -- | The fixed point of a monadic computation.
57 -- @'mfix' f@ executes the action @f@ only once, with the eventual
58 -- output fed back as the input. Hence @f@ should not be strict,
59 -- for then @'mfix' f@ would diverge.
60 mfix :: (a -> m a) -> m a
61 #endif /* !__HUGS__ */
63 -- Instances of MonadFix for Prelude monads
66 instance MonadFix Maybe where
67 mfix f = let a = f (unJust a) in a
68 where unJust (Just x) = x
69 unJust Nothing = error "mfix Maybe: Nothing"
72 instance MonadFix [] where
73 mfix f = case fix (f . head) of
75 (x:_) -> x : mfix (tail . f)
78 instance MonadFix IO where
81 instance MonadFix ((->) r) where
82 mfix f = \ r -> let a = f a r in a
84 #if defined(__GLASGOW_HASKELL__)
85 instance MonadFix (ST s) where