[GHC] #6018: Injective type families

[GHC]

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

 Richard, I don't understand your terminology here (though I do understand
 the `F` example):

 > The equations defining a head-injective function all have RHSs with
 different heads. What appears below the heads does not matter.

 Could you explain what you mean by `RHSs with different heads` and `below
 the heads`? Up till now I thought that the term "head" refers to `type
 family F a` part of the declaration.

 I also don't understand why `G` is not head-injective, but perhaps this
 will become clear to me once earlier questions are answered.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:50>
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