[GHC] #12792: Wrong error message when using a data type as a class instance head

31 Oct 2016

      #12792: Wrong error message when using a data type as a class instance head
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           Reporter:  nomeata        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:  newcomer       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 Prelude> data List a = Nil | Cons a (List a)
 Prelude> instance List a

 <interactive>:3:10: error:
     • Expected a constraint, but ‘List a’ has kind ‘*’
     • In the instance declaration for ‘List a’
 Prelude> instance List a where foo = foo

 <interactive>:2:23: error:
     ‘foo’ is not a (visible) method of class ‘List’
 }}}

 Clearly, the constraint check should happen before we look at the method
 names.

 Happens with 7.10 and 8.0.1 and HEAD.

 Shouldn’t be hard, marking at newcomer.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12792
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