After reading (and skimming some sections of) Wadler's paper on monads, I decided to work through the paper more closely, defining functor and applicative instances for each of the monads he describes. Using the type synonym
type M a = State -> (a, State)
type State = Int
Wadler uses to define the state monad, I have the following (using related names so I can define them with a newtype declaration later on).
fmap' :: (a -> b) -> M a -> M b
fmap' f m = \st -> let (a, s) = m st in (f a, s)
pure' :: a -> M a
pure' a = \st -> (a, st)
(<@>) :: M (a -> b) -> M a -> M b
sf <@> sv = \st -> let (f, st1) = sf st
(a, st2) = sv st1
in (f a, st2)
return' :: a -> M a
return' a = pure' a
bind :: M a -> (a -> M b) -> M b
m `bind` f = \st -> let (a, st1) = m st
(b, st2) = f a st1
in (b, st2)
When I switch to using a type constructor in a newtype declaration, e.g.,
newtype S a = S (State -> (a, State))
everything falls apart. Everything is just a slight modification, for instance,
instance Functor S where
fmap f (S m) = S (\st -> let (a, s) = m st in (f a, s))
instance Applicative S where
pure a = S (\st -> (a, st))
however nothing runs in GHC due to the fact that the lambda expression is hidden inside that type constructor. Now the only solution I see is to define a function:
isntThisAnnoying s (S m) = m s
in order to bind s to 'st' and actually return a value, e.g.,
fmap f m = S (\st -> let (a, s) = isntThisAnnoying st m in (f a, s))
Is there another way to do this that doesn't use these auxiliary functions?