[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):

 This idea seems reasonable... but your example does not, I'm afraid. If
 `T` and `S` are datatypes, then how could they be equal? And your `F Int ~
 S` seems like it could just be an instance of the type family. On the
 other hand

 {{{
 data T
 data S

 type family F a
 type family G a

 instance F T ~ G S
 }}}

 seems more realistic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12680#comment:1>
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