[GHC] #9171: Clarify error messages when there is a kind mismatch

[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):

 I've written several type families where we really do want this behavior.
 I can't speak to ''why'' it was designed to work this way, but I have
 benefitted from this decision.

 One example is to be able to encode overloaded numbers in types:

 {{{
 type family FromNat (n :: Nat) :: k
 type instance FromNat n = n        -- identity
 type instance FromNat n = ToU n    -- for UnaryNat
 type instance FromNat n = ToZ n    -- for Z (integers)

 data UnaryNat = Zero | Succ UnaryNat
 type family ToU n where
   ToU 0 = Zero
   ToU n = Succ (ToU (n - 1))

 data Z = ...
 type family ToZ n where ...
 }}}

 I admit that I'm probably in the minority at using this feature, but it
 ''is'' being used. On the other hand, it is a little alarming to most
 users at first, and it's definitely not the most intuitive feature.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9171#comment:6>
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