[GHC] #12680: Permit type equality instances in signatures

[GHC]

#12680: Permit type equality instances in signatures
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I've not followed the Backpack blow-by-blow, but using `data S` to denote
 an abstract type of any stripe looks bad to me. `type S` is much better,
 because users can't make any assumptions about a `type` declaration that
 they might for a `data` declaration.

 Separately, you could get yourself into deep water here (not that that's
 necessary bad):

 {{{
 type Nat

 type family (a :: Nat) + (b :: Nat)
 instance a + b ~ b + a
 }}}

 Good luck implementing this signature! :)

 It would be super awesome if the user could prove the equality manually.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12680#comment:3>
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