[GHC] #12422: Add decidable equality class

[GHC]

#12422: Add decidable equality class
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 There are several fun things to respond to here!

 1. Making `:~:` heterogeneous would likely break client code. The problem
 is that, right now, if I say `(Int :~: a)` somewhere, GHC can know that `a
 :: Type`. If we make `:~:` hetero, then GHC would rightly have `a :: k`,
 which can cause trouble. This is not hypothetical: I tried making this
 change once upon a time, and things broke. Even worse, it was hard to go
 from the error message reported to the actual cause. I advocate against
 this change.

 2. We should have `:~~:` in the standard library. But I was too exhausted
 from implementing `TypeInType` to start this debate on the libraries
 mailing list. We should really put it in for 8.2.

 3. Here is the hetero version of `TestEquality`:

 {{{
 -- hetero
 class HTestEquality (f :: forall k. k -> Type) where
   hTestEquality :: f a -> f b -> Maybe (a :~~: b)
 }}}

     Note that `f` has to be polymorphic in its kind to avoid constraining
 `a` and `b` to have the same kind. Yes, this is a higher-rank kind!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12422#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler