{-# LANGUAGE CPP #-}
#include "containers.h"
{-# OPTIONS_HADDOCK hide #-}
module Utils.Containers.Internal.State where
import Prelude hiding (
#if MIN_VERSION_base(4,8,0)
Applicative
#endif
)
import Control.Monad (ap)
import Control.Applicative (Applicative(..), liftA)
newtype State s a = State {runState :: s -> (s, a)}
instance Functor (State s) where
fmap = liftA
instance Monad (State s) where
{-# INLINE return #-}
{-# INLINE (>>=) #-}
return = pure
m >>= k = State $ \ s -> case runState m s of
(s', x) -> runState (k x) s'
instance Applicative (State s) where
{-# INLINE pure #-}
pure x = State $ \ s -> (s, x)
(<*>) = ap
execState :: State s a -> s -> a
execState m x = snd (runState m x)