[Haskell-cafe] New type of ($) operator in GHC 8.0 is problematic

[Ben Gamari]

Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> writes:

> On Fri, Feb 05, 2016 at 07:19:25PM +0000, Tom Ellis wrote:
>> On Fri, Feb 05, 2016 at 01:13:23PM -0500, Richard Eisenberg wrote:
>> > We're in a bit of a bind in all this. We really need the fancy type for
>> > ($) so that it can be used in all situations where it is used currently. 
>> 
>> Is there a list of situations where ($) is used currently that give rise to
>> this need?
>
> Does anyone have any idea about this?  What is it about ($) that means it
> needs a new funky type whereas (apparently) nothing else does?

The first (albeit rather unconvincing) example I can think of is be
something like,

    getI# :: Int -> Int#
    getI# (I# n#) = n#

    n# :: Int#
    n# = getI# $ 5 + 8

Richard likely has something a bit less contrived though.

This does raise the question of why ($) is generalized, yet (.) is not,

    (.) :: forall (l :: Levity) a b (c :: TYPE l).
           (b -> c) -> (a -> b) -> (a -> c)
    (.) f g x = f (g x)

Cheers,

- Ben
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