[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
Differential Revisions:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |         Test Case:
            Difficulty:  Unknown     |          Blocking:
            Blocked By:              |
       Related Tickets:  #4259       |
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Comment (by goldfire):

 Replying to [comment:41 jstolarek]:
 > This is beyond my knowledge - I would require some guidance here. But if
 it's possible to implement injective type families without this change
 then I would postpone it until later and possibly consider it a separate
 task.

 Yes, it's possible, but there are some circumstances in which users could
 say that GHC gives an erroneous type error, in that GHC would manifestly
 have enough knowledge to make progress but doesn't. I agree that you
 should go ahead without this piece, though!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:42>
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