module Moi where

newtype Moi s a = Moi { runMoi :: s -> (a, s) }

instance Functor (Moi s) where
    -- fmap :: (a -> b) -> Moi s a -> Moi s b
    fmap f (Moi g) = Moi (\x -> h $ g x)
        where h (a, b) = (f a, b)

instance Applicative (Moi s) where
    -- pure :: a -> Moi s a
    pure a = Moi (\s -> (a, s))
    -- (<*>) :: Moi s (a -> b) -> Moi s a -> Moi s b
    (Moi f) <*> (Moi g) = Moi (\s -> ((fst (f s)) (fst (g s)), s))

instance Monad (Moi s) where
    return = pure
    -- (>>=) :: Moi s a -> (a -> Moi s b) -> Moi s b
    (Moi f) >>= g = Moi (\s -> (runMoi (g (fst (f s)))) (snd (f s)))

get :: Moi s s
get = Moi $ \a -> (a, a)

put :: s -> Moi s ()
put s = Moi $ \_ -> ((), s)

exec :: Moi s a -> s -> s
exec (Moi sa) s = snd (sa s)

eval :: Moi s a -> s -> a
eval (Moi sa) s = fst (sa s)

modify :: (s -> s) -> Moi s ()
modify f = Moi $ \a -> ((), f a)