Control.Monad.Fix

Monadic fixpoints.

For a detailed discussion, see Levent Erkok's thesis, Value Recursion in Monadic Computations, Oregon Graduate Institute, 2002.

Documentation

class Monad m => MonadFix m where

Monads having fixed points with a 'knot-tying' semantics. Instances of MonadFix should satisfy the following 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)

This class is used in the translation of the recursive do notation supported by GHC and Hugs.

Methods

mfix :: (a -> m a) -> m a

The fixed point of a monadic computation. mfix f executes the action f only once, with the eventual output fed back as the input. Hence f should not be strict, for then mfix f would diverge.

Instances

MonadFix []

Methods

mfix :: (a -> [a]) -> [a]

MonadFix IO

Methods

mfix :: (a -> IO a) -> IO a

MonadFix Maybe

Methods

mfix :: (a -> Maybe a) -> Maybe a

MonadFix Identity

Methods

mfix :: (a -> Identity a) -> Identity a

MonadFix ((->) r)

Methods

mfix :: (a -> (->) r a) -> (->) r a

MonadFix (Either e)

Methods

mfix :: (a -> Either e a) -> Either e a

MonadFix (ST s)

Methods

mfix :: (a -> ST s a) -> ST s a

MonadFix (ST s)

Methods

mfix :: (a -> ST s a) -> ST s a

fix :: (a -> a) -> a

fix f is the least fixed point of the function f, i.e. the least defined x such that f x = x.