[GHC] #12114: Make injectivity check less conservative (was: GHC rejects injective type family)

[GHC]

#12114: Make injectivity check less conservative
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        Reporter:  MikeIzbicki       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
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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Changes (by goldfire):

 * type:  bug => feature request


Comment:

 I agree completely with your analysis. The problem here is that for GHC to
 perform this analysis, it would have to characterize the set of possible
 outputs from a function. While this may be possible, this pushes us into a
 much more involved injectivity check than we currently have.

 I am thus relabeling this ticket as a feature request. However, without a
 suggested algorithm, it's hard to know how to make progress here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12114#comment:1>
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