How can I declare the type of a function defined within a let

Question

I want to write something like:

f :: (a -> b) -> a -> c -> b
f g =
   let inner :: a -> c -> b
       inner x y = g x
   in inner

but this gives me an error.because it doesn't recognize that I'm trying to refer to the same "a" and "b" types as in the declaration as f

How can I explicitly give the proper type for inner?

Answer 1

You'll need the extension ScopedTypeVariables. You also need to add an explicit forall a b c . to your signature, which signals to bind the variables for the whole scope of the definition.

{-# LANGUAGE ScopedTypeVariables #-}

f :: forall a b c . (a -> b) -> a -> c -> b
f g =
   let inner :: a -> c -> b
       inner x y = g x
   in inner

Answer 2

One way to do this would be to bind the outer and inner types by making g a parameter in inner, like this....

f g =
   let inner :: (a->b)->a -> c -> b
       inner g x y = g x
   in inner g

This does change your structure slightly though.... And possibly negates the reason to have an inner let in the first place, but in many cases (depending on the larger program) this might help.