```{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}

-----------------------------------------------------------------------------
-- |
-- Copyright   :  (c) Andy Gill 2001,
--                (c) Oregon Graduate Institute of Science and Technology, 2002
-- Stability   :  experimental
-- Portability :  portable
--
--
-- For a detailed discussion, see Levent Erkok's thesis,
--
-----------------------------------------------------------------------------

fix
) where

import Data.Either
import Data.Function ( fix )
import Data.Maybe
import GHC.Base ( Monad, error, (.) )
import GHC.List ( head, tail )
import GHC.ST
import System.IO

-- | 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' ('Control.Monad.liftM' h . f)  =  'Control.Monad.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.
-- | 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.
mfix :: (a -> m a) -> m a

mfix f = let a = f (unJust a) in a
where unJust (Just x) = x
unJust Nothing  = error "mfix Maybe: Nothing"

mfix f = case fix (f . head) of
[]    -> []
(x:_) -> x : mfix (tail . f)