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

To specify only the type of an argument, you can write something like

fn list = 2 * length list
  where
    a :: [Char]
    a = list `asTypeOf` a

So that it is easy to later amend it like, e.g.,

fn list = 2 * fromIntegral (length list)
  where
    a :: [Char]
    a = list `asTypeOf` a

and have its inferred type change accordingly:

*Main> :t fn
fn :: [Char] -> Int
*Main> :r
-- changed file reloaded
*Main> :t fn
fn :: (Num t) => [Char] -> t

You could use the same contorted technique to specify the return type of a function, perhaps defined in pointfree style, but this is not pretty.

fn2 list = r
  where
    r :: Int
    r = f list
    f = (2 *) . length

It is also not much different from what you have right now, just keeps the code and the type spec separated.