[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 may have said this elsewhere, but if we keep the "untouchable" stuff in
 the error message, it would be great to include a link to more
 information. Untouchable variables are really subtle, and an understanding
 of them is necessary for a programmer to make progress here. Section 5.2
 of the !OutsideIn paper is a great starting place for what should be at
 the other end of the link.

 Personally, I favor the second message above -- it has more information a
 programmer might use to fix the problem.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9223#comment:3>
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