[GHC] #8506: misleading error message for duplicate type class instances

[GHC]

#8506: misleading error message for duplicate type class instances
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        Reporter:  carter                    |            Owner:
            Type:  bug                       |           Status:  closed
        Priority:  normal                    |        Milestone:
       Component:  Compiler (Type checker)   |          Version:  7.6.3
      Resolution:  fixed                     |         Keywords:
Operating System:  Unknown/Multiple          |     Architecture:
 Type of failure:  None/Unknown              |  Unknown/Multiple
       Test Case:  parser/should_fail/T8506  |       Difficulty:  Unknown
        Blocking:                            |       Blocked By:
                                             |  Related Tickets:
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Comment (by carter):

 awesome, thanks!

 I merely wanted to highlight the distinction of
 {{{



 src/Numerical/Types/Shape.hs:97:17:
     Unexpected type `Z' where type variable expected
     In the declaration of `Shape Z'

 }}}

  when theres a data family in the class

 vs

 {{{
 src/Numerical/Types/Shape.hs:88:16:
     Unexpected type `Z' where type variable expected
     In the declaration of `Shapable Z'

 }}}

 because it wasn't clear to me, and that was part of why the original
 original snippet was especially confusing. Namely that it seemed to be
 saying I couldn't instantiate a data family  (in addition to the whole
 typeclass dec needs to have variables as parameters)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8506#comment:8>
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