[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:  v0d1ch
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       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:  #13192            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 > How would I proceed further with these informations ?

 Not quite sure how to answer this. You might need to continue tracing the
 code path into `tcMatchTys`.

 >
 > I am also wondering if I should introduce new type that will give us the
 option to customize failure message instead of just `Nothing` ?

 Yes, I think so. For example, instead of `Nothing`, maybe it would work
 well to return an `Int` that gives the index of the first argument that
 mismatches. Then, the error-message generator could see if that index
 corresponds to an invisible argument; if so, suggest `-fprint-explicit-
 kinds`. Perhaps there's a better design, as well.

 >

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789#comment:20>
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