[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Replying to [comment:8 simonpj]:
 > It's simple. Consider
 > {{{
 > class C v where
 >   type T a b v cd
 > }}}
 > In any instance declaration, the type in the `v` position of the `type`
 instance must be the same as in the instance header:
 > {{{
 > instance ... => C ty where
 >   type T p q ty r s = <rhs>
 > }}}
 > I think we do (or at least should) also insist that `p`, `q`, `r`, `s`
 are distinct type variables, otherwise the type instance is more specific
 than the class instance, but that's a separate matter.

 We don't and I don't think we should. Isn't it okay to define multiple
 instances that refine the other parameters, like this (currently
 accepted)?
 {{{
 class C v where
   type T v w

 instance C (Either a b) where
   type T (Either a b) Int = a
   type T (Either a b) Char = b
 }}}
 In fact you're not obliged to provide any equations for `T` at all (though
 we do warn in that case).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:13>
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