[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 simonpj):

 That would also make sense.  So you would also like to be able to write
 {{{
 class C v where
   type T v

 instance C (Either a b) where
   type T (Either x y) = y -> x
 }}}
 But I think the current intent is "equal" not "equal up to alpha
 equivalence".

 Which do people like best?  I in the class decl you obviously must use the
 same `v`; that's how you link the associated type to its class.  In the
 instance we could loosen it.

 It's a usability issue not a technical one.  Which is best?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:4>
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