[GHC] #15119: Program involving existentials and type class constraint gets rejected
GHC
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Fri May 4 12:37:11 UTC 2018
#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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