[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 MikeIzbicki):

 Replying to [comment:1 kosmikus]:

 I was under the impression that type families had to be functions, and so:

 {{{
 type family GetParam (p::k1) (t::k2) :: k3
 type instance GetParam Base Int   = Int
 type instance GetParam Base Int   = Maybe
 }}}

 would be rejected for having conflicting definitions.  If we change the
 code to:

 {{{
 type family GetParam (p::k1) (t::k2) :: *
 type instance GetParam Base Int   = Int
 type instance GetParam Base Int   = Double
 }}}

 then the program does get rejected as having conflicting definitions.  Is
 there a good reason to allow the former but not the latter?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9171#comment:3>
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