[GHC] #10760: Conflicting typefamily instances not reported with polykinds

[GHC]

#10760: Conflicting typefamily instances not reported with polykinds
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        Reporter:  steely            |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:  invalid           |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by steely):

 I understand. This is indeed very subtle and ought to be documented.
 I am not sure whether this explanation should go in the section about type
 families alone, since this issue is
 related to their interaction with PolyKinds, but anyway it would be more
 precise to include something like:

 In the paragraph about "Type Family Declarations".
 The result kind annotation is optional and, as usual, defaults to * if
 omitted. Polykinded type families can be
 declared using a parameter in the kind annotation:

 {{{
 type family F a :: k
 }}}

 Note that in this case the kind parameter k is an actual implicit
 parameter of the type family and as such is relevant
 when checking the apartness of type family equations.

 In the paragraph about "Compatibility and apartness of type family
 equations'":

 1. all corresponding types and '''implicit kinds''' in the patterns are
 apart.

 The example of this ticket could also be included to show this.

 I am not sure whether in GHC 7.10 kind variables have to be explicitly
 given in type families, making it more intuitive.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10760#comment:2>
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