{-# LANGUAGE Trustworthy #-} {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeOperators #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Monad.Fix -- Copyright : (c) Andy Gill 2001, -- (c) Oregon Graduate Institute of Science and Technology, 2002 -- License : BSD-style (see the file libraries/base/LICENSE) -- Maintainer : libraries@haskell.org -- Stability : experimental -- Portability : portable -- -- Monadic fixpoints. -- -- For a detailed discussion, see Levent Erkok's thesis, -- /Value Recursion in Monadic Computations/, Oregon Graduate Institute, 2002. -- ----------------------------------------------------------------------------- module Control.Monad.Fix ( MonadFix(mfix), fix ) where import Data.Either import Data.Function ( fix ) import Data.Maybe import Data.Monoid ( Dual(..), Sum(..), Product(..) , First(..), Last(..), Alt(..) ) import GHC.Base ( Monad, errorWithoutStackTrace, (.) ) import GHC.Generics 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. class (Monad m) => MonadFix m where -- | 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 -- Instances of MonadFix for Prelude monads -- | @since 2.01 instance MonadFix Maybe where mfix f = let a = f (unJust a) in a where unJust (Just x) = x unJust Nothing = errorWithoutStackTrace "mfix Maybe: Nothing" -- | @since 2.01 instance MonadFix [] where mfix f = case fix (f . head) of [] -> [] (x:_) -> x : mfix (tail . f) -- | @since 2.01 instance MonadFix IO where mfix = fixIO -- | @since 2.01 instance MonadFix ((->) r) where mfix f = \ r -> let a = f a r in a -- | @since 4.3.0.0 instance MonadFix (Either e) where mfix f = let a = f (unRight a) in a where unRight (Right x) = x unRight (Left _) = errorWithoutStackTrace "mfix Either: Left" -- | @since 2.01 instance MonadFix (ST s) where mfix = fixST -- Instances of Data.Monoid wrappers -- | @since 4.8.0.0 instance MonadFix Dual where mfix f = Dual (fix (getDual . f)) -- | @since 4.8.0.0 instance MonadFix Sum where mfix f = Sum (fix (getSum . f)) -- | @since 4.8.0.0 instance MonadFix Product where mfix f = Product (fix (getProduct . f)) -- | @since 4.8.0.0 instance MonadFix First where mfix f = First (mfix (getFirst . f)) -- | @since 4.8.0.0 instance MonadFix Last where mfix f = Last (mfix (getLast . f)) -- | @since 4.8.0.0 instance MonadFix f => MonadFix (Alt f) where mfix f = Alt (mfix (getAlt . f)) -- Instances for GHC.Generics -- | @since 4.9.0.0 instance MonadFix Par1 where mfix f = Par1 (fix (unPar1 . f)) -- | @since 4.9.0.0 instance MonadFix f => MonadFix (Rec1 f) where mfix f = Rec1 (mfix (unRec1 . f)) -- | @since 4.9.0.0 instance MonadFix f => MonadFix (M1 i c f) where mfix f = M1 (mfix (unM1. f)) -- | @since 4.9.0.0 instance (MonadFix f, MonadFix g) => MonadFix (f :*: g) where mfix f = (mfix (fstP . f)) :*: (mfix (sndP . f)) where fstP (a :*: _) = a sndP (_ :*: b) = b