[GHC] #15119: Program involving existentials and type class constraint gets rejected

[GHC]

#15119: Program involving existentials and type class constraint gets rejected
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        Reporter:  sgraf             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Right, you're going to need //something// which determines `a`, be it
 `Proxy`, `Tagged`, or something else. There may come a day in which you
 can write:

 {{{#!hs
 forall a -> C a => Int
 }}}

 That is, where one can make `a` a visible argument (i.e., a "big lambda",
 as you've put it). But until then, you'll have to resort to baser tricks
 to accomplish what you seek.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15119#comment:3>
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