[GHC] #6018: Injective type families

[GHC]

#6018: Injective type families
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        Reporter:  lunaris     |            Owner:  simonpj
            Type:  feature     |           Status:  new
  request                      |        Milestone:  7.10.1
        Priority:  normal      |          Version:  7.4.1
       Component:  Compiler    |         Keywords:  TypeFamilies, Injective
      Resolution:              |     Architecture:  Unknown/Multiple
Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple             |       Blocked By:
 Type of failure:              |  Related Tickets:  #4259
  None/Unknown                 |
       Test Case:              |
        Blocking:              |
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Comment (by acowley):

 My intended uses seem to arise quite often, but I've not found one that
 boils down into a small, self-contained, compelling example. In broad
 terms, I would like to use type families for decoupling, as hpacheco
 describes, and to do things like move back and forth between a set of
 distinct types and singleton Nats.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:28>
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