[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:  #11449, #11451    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:17 goldfire]:
 > Re comment:16: But your two wildcards are referring to the same type. I
 think we should require a name here, because the name is actually used
 twice.

 I suppose if we take interpret every occurrence of `_` as a fresh type
 variable, then the `Either a _` in the instance head cannot be the same as
 the one in the associated type family instance.

 And there's even another reason to require named type variables: what
 about `-XInstanceSigs`? If you allowed wildcards in instances, you could
 have a scenario like this:

 {{{#!hs
 class C a where
   c :: a -> String

 instance C (Maybe _) where
   c :: Maybe _ -> String
   c _ = "huh?"
 }}}

 And we certainly don't allow wildcards in term-level type signatures like
 this.

 In light of this, I retract my objection in comment:16. As Simon said, the
 description of the ticket is still right.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:18>
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