[GHC] #9171: Clarify error messages when there is a kind mismatch (was: Can't match type family with kind polymorphism)

[GHC]

#9171: Clarify error messages when there is a kind mismatch
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        Reporter:  MikeIzbicki                |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.8.2
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by goldfire):

 kosmikus is right -- kind variables are properly seen as arguments to a
 type family, and the OP's code is correctly rejected.

 However, the error message is quite suboptimal. I wonder if there's a way
 to detect that kinds are at issue and to suggest `-fprint-explicit-kinds`
 (which would make the problem more apparent). As a strawman proposal, we
 could just look at the rendered output; if the "expected" and "actual" are
 identical, suggest `-fprint-explicit-kinds`.

 Leaving this ticket open as a request to fine-tune error message.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9171#comment:2>
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