Question

I want to access type variables in an instance, that don’t show up in the instance’s parameters. For example,

class A a where foo :: a b
data C a
instance A C where
    foo :: forall b. C b
    foo = undefined :: C b

Of course, the above will resolve without the scoped type expression, but I have a non-toy example where I actually want it.

edit

please try your code before pasting it as an answer! The above (Daniel’s answer) results in

Test.hs:51:5: Misplaced type signature: foo :: forall b. C b
Failed, modules loaded: none.

Answer 1

I think to overcome the problem with Daniel’s solution that you are not allowed to provide a signature for a type class function you can simply define a top-level function and “rename” it in the instance of the type class. In your simple example the following should work.

{-# LANGUAGE ScopedTypeVariables #-}

class A a where 
    foo :: a b

instance A C where
    foo = foo'

foo' :: C b
foo' = undefined :: C b