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{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE PatternSynonyms #-}
module GHC.Utils.Monad.State where
import GHC.Prelude
import GHC.Exts (oneShot)
newtype State s a = State' { runState' :: s -> (# a, s #) }
deriving (Functor)
pattern State :: (s -> (# a, s #))
-> State s a
-- This pattern synonym makes the monad eta-expand,
-- which as a very beneficial effect on compiler performance
-- See #18202.
-- See Note [The one-shot state monad trick] in GHC.Utils.Monad
pattern State m <- State' m
where
State m = State' (oneShot $ \s -> m s)
instance Applicative (State s) where
pure x = State $ \s -> (# x, s #)
m <*> n = State $ \s -> case runState' m s of
(# f, s' #) -> case runState' n s' of
(# x, s'' #) -> (# f x, s'' #)
instance Monad (State s) where
m >>= n = State $ \s -> case runState' m s of
(# r, s' #) -> runState' (n r) s'
get :: State s s
get = State $ \s -> (# s, s #)
gets :: (s -> a) -> State s a
gets f = State $ \s -> (# f s, s #)
put :: s -> State s ()
put s' = State $ \_ -> (# (), s' #)
modify :: (s -> s) -> State s ()
modify f = State $ \s -> (# (), f s #)
evalState :: State s a -> s -> a
evalState s i = case runState' s i of
(# a, _ #) -> a
execState :: State s a -> s -> s
execState s i = case runState' s i of
(# _, s' #) -> s'
runState :: State s a -> s -> (a, s)
runState s i = case runState' s i of
(# a, s' #) -> (a, s')
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