[GHC] #10586: GHC 7.10.1 panic due to wildcard in data family instance

[GHC]

#10586: GHC 7.10.1 panic due to wildcard in data family instance
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        Reporter:  WrenThornton      |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:  fixed             |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #3699             |  Differential Revisions:
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Comment (by simonpj):

 > I'm actually thinking about adding support for wild cards in data/type
 instances. I would just replace them with fresh meta variables. This would
 also address #3699. I don't think named wild cards or extra-constraints
 wild cards make sense here. What do you think?

 Yes, I agree.  Do not use a ''meta'' variable.  These are really
 ''skolems'', just like the named type variables in an ordinary
 declaration, just anonymous ones.

 Would need documentation about exactly where these skolem wildcards can
 appear.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10586#comment:7>
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