[GHC] #15691: Marking Pred(S n) = n as injective

[GHC]

#15691: Marking Pred(S n) = n as injective
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:  invalid           |  InjectiveFamilies
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):

 There are no current plans to implement "Constrained Type Families". The
 natural way to do it requires dependent types (because it requires the
 appearance of dictionaries, which are terms, in types). We could hack
 something together that didn't use dependent types, but it would be a
 hack.

 I would love to see Constrained Type Families indeed implemented once we
 have dependent types.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15691#comment:2>
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