Suppose we have the following newtype definition:
newtype A = A { _run :: Monad m => A -> [Int] -> m Int }
This does not compile with GHC 8.0.2:
error: Not in scope: type variable ‘m’
Replacing m with a concrete typeclass like IO or [] does compile, as I would expect. Given that this is ok, why does GHC not allow the signature above? What is wrong with adding a typeclass constraint inside of this newtype?
This is possible:
{-# LANGUAGE RankNTypes #-}
newtype A = A { _run :: forall m. Monad m => A -> [Int] -> m Int }
It's hard to tell what you want to do, but this isn't very usable. Any value of type A needs to work for all monads (you don't get to choose).
This is also possible, with the same restrictions:
{-# LANGUAGE GADTs #-}
data A where A :: Monad m => (A -> [Int] -> m Int) -> A
But perhaps you mean something more like
newtype A m = A { _run :: A m -> [Int] -> m Int }
This allows for values of different types of A using different monads.