[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 (Type checker)  |          Version:  7.8.2
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by goldfire):

 I'm a little confused about what's going on about the core complaint, but,
 tangentially, it seems to me that calling `a` a "rigid type variable" is
 wrong here. Isn't `a` a metavariable, especially if it's untouchable?

 I should say that blindly following the rules from !OutsideIn about
 untouchable variables does lead me to a place of unifying `m a` with `m
 ()` where `a` is untouchable. But, something is still fishy about it that
 I can't quite pin down. In other cases, when something is untouchable, I
 can construct a way in which the type in question is not a principal
 type... and I can't quite do this here, hence my confusion.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9223#comment:1>
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