[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 v0d1ch):

 I have determined that in the failure case in this specific example we
 want the `Role` to be `Nominal` and we get a `[Type]` that looks like
 this:

 `[* -> *, Maybe]`


 while the `TyCon` is `F`

 How would I precede further with these informations ?

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

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789#comment:19>
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