On Fri, Apr 11, 2014 at 12:28 AM, Jason Dagit <dagitj@gmail.com> wrote:

It seems to be an issue with giving everything on a single line:

Prelude ë> let whee x = x == x && null (show x) :: Show a => Eq a => a => Bool

<interactive>:2:14:
Couldn't match expected type `a -> Bool' with actual type `Bool'
In the expression:

x == x && null (show x) :: Show a => Eq a => a => Bool

In an equation for `whee':
whee x = x == x && null (show x) :: Show a => Eq a => a => Bool

As the error message indicates, the (:: type) is being applied to the open term: x == x && null (show x), and not to the function whee.

To do this single-line def properly, I think you need -XScopedTypeVariables:

let whee (x :: a) = x == x && null (show x) :: Show a => Eq a => Bool

I see now to make it work as a one liner I could do it this way:

Prelude ë> :t (\x -> x == x && null (show x)) :: Show a => Eq a => a => Bool
(\x -> x == x && null (show x)) :: Show a => Eq a => a => Bool

:: (Eq a, Show a) => a -> Bool

And to recover the named binding:

let whee = (\x -> x == x && null (show x)) :: Show a => Eq a => a => Bool

-- Kim-Ee