[GHC] #9223: Type equality makes type variable untouchable

[GHC]

#9223: Type equality makes type variable untouchable
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              Reporter:  Feuerbach   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by tvynr):

 I'm glad the example is at least somewhat useful.  :)  I'm unfamiliar
 enough with the mechanics involved that I don't know if this is related.
 Should I open another ticket instead or just leave this stuff here?

 The thing that seems especially peculiar to me is that, in the example
 above, GHC 7.6.3 can infer the type just fine.  This suggests that the GHC
 7.8.3 inference algorithm has been made more conservative in some way.
 I'm guessing that this difference is related to fixing a bug of some kind
 but, as you say, this case only seems to have the one type which could be
 inferred.

 In the meanwhile, I suppose we'll just use the workaround of an explicit
 unwrapper function if we need to move to GHC 7.8.  Thanks!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9223#comment:6>
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