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

21 Jan 2016

      #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 RyanGlScott):

 My question is: what exactly do we mean by "require the variables to be
 the same"? For example, would the following instance be rejected under
 this new rule?

 {{{#!hs
 class C v where
   type T v

 instance C (Either a b) where
   type T (Either _ b) = b
 }}}

 Also, what rules (if any) do we apply to non-linear type variables (like
 in the example above)?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11450#comment:7
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