[GHC] #8177: Roles for type families

[GHC]

#8177: Roles for type families
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        Reporter:  simonpj           |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:  TypeFamilies
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 glguy):

 I'd like to share my usecase for wanting this feature

 Given the following type:

 {{{#!haskell
 -- | Family of N-ary operator types.
 type family Op n a b where
   Op 'Z     a b = b
   Op ('S n) a b = a -> Op n a b
 }}}

 I'd love to be able to

 {{{#!haskell
 coerce :: Coercible a b => Op n a c -> Op n b c
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177#comment:32>
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