[Haskell-beginners] Applicative for State

[mike h]

Hi,

I have a State by another name, Stat,  just to experiment and learn.

newtype Stat s a = Stat { runStat :: s -> (a, s) }

instance Functor (Stat s) where
    fmap f (Stat g) = Stat $ \s -> (f a, s) where 
        (a, s) =  g s
              

instance Applicative (Stat s) where
    pure a = Stat $ \s -> (a, s)

    (Stat f) <*> (Stat g) = Stat $ \s -> (a, s) where 
        (a, s) = undefined

I really can’t get what the <*> in the Applicative should be!
I just do see how I ‘get the f out of the Stat’ and then apply it.

I’d be really grateful if someone would explain what it should be and the steps/reasoning needed to get there.


Many thanks

Mike