src/Utils/Containers/Internal/State.hs

    {-# LANGUAGE CPP #-}
    #include "containers.h"
    {-# OPTIONS_HADDOCK hide #-}

    -- | A clone of Control.Monad.State.Strict.
    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)