How to define function signatures partially in Haskell?

Question

Starting point:

fn :: [a] -> Int
fn = (2 *) . length

Let\'s say we only want to constrain the return value, then we could write:

Answer 1

I've been looking for a way to say 'x's type unifies with T'. The solutions given by Will Ness and chi are close to what I came up with, but there is a way to do it in Haskell 98, without butchering your own function.

-- Your function, without type signature.
fn = (2 *) . length

-- The type signature, without actual definition.
fnTy :: [Char] -> a
fnTy = undefined

-- If this type checks, then the type of 'fn' can be unified 
--                                      with the type of 'fnTy'.
fn_unifies_with_type :: ()
fn_unifies_with_type = let _ = fn `asTypeOf` fnTy in ()

You could even go for just

fn = (2 *) . length
  where
    _ = fn `asTypeOf` (undefined :: [Char] -> a)