[GHC] #10789: Notify user when a kind mismatch holds up a type family reduction

[GHC]

#10789: Notify user when a kind mismatch holds up a type family reduction
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:  newcomer,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 A word of warning: TcErrors bears the accumulation of small patches that
 have been applied to incrementally improve error messages. I, personally,
 always find the module hard to navigate. However, if you have a specific
 erroneous situation in mind, it's not that hard to trace how that
 particular error filters through the module, and you should be able to
 find the stretch of code where the error itself is rendered. (A good
 starting place is to search for the text printed in the current error
 message.)

 To fix this, you'll have to add an extra check somewhere and then tailor
 the error message if necessary.

 Give a holler if you get stuck!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789#comment:12>
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