[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 goldfire):

 Replying to [comment:50 jstolarek]:
 > 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.

 Here, "head" means the head of the RHS type, not the type declaration --
 it's the root of the type's AST. Does that help?

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

 `G` isn't head-injective because both RHSs are headed by `Just`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:52>
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