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Control.Monad.Fix

Portability | portable
Stability | experimental
Maintainer | libraries@haskell.org

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

class Monad m => MonadFix m where

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

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

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 IO
MonadFix Maybe
MonadFix []
MonadFix (ST s)
MonadFix (ST s)

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`.
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