[GHC] #11385: Unify named wildcards in different type applications

[GHC]

#11385: Unify named wildcards in different type applications
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:
      Resolution:                    |  NamedWildCards TypeApplications
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 goldfire):

 Just seeing this now (missed it the first time it went through my inbox).

 I'm uncertain about this one. The current behavior is in line with the
 documentation, in that both arguments to, e.g., `const` are ''different''
 types. So I think it's behaving according to spec.

 Should the spec change? Perhaps. If I recall correctly, there was a long
 and tortuous debate about the scoping of named wildcards, with all sides
 having good arguments in their favor. The nice thing about the current
 rule is that it's exceedingly simple and easy to predict. With your
 proposed rule, then you could write `f @_a (<something long>) @_a` and
 have the two `_a`s share a scope. So I think I've convinced myself to lean
 against this idea, but I'm open to argument.

 A better solution is to allow visible type ''abstraction'':

 {{{
 asTypeOf = \ @a -> const @a @a
 }}}

 That way, you can say exactly what you mean.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11385#comment:2>
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