[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        |            Status:  new
  request                            |
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 David Christiansen requested this, and I want to record it so I don't
 forget. What he wants to be able to do is say things like this in
 signatures:

 {{{
 signature A where

 data T
 data S
 type family F a

 instance T ~ S
 instance F Int ~ S
 }}}

 This is currently rejected by GHC because you are not allowed to define
 instances of type equality. But really all this is saying is that the type
 equality must hold when we eventually implement the module; easy to check.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12680>
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