[GHC] #9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving
GHC
ghc-devs at haskell.org
Thu Jun 28 08:10:53 UTC 2018
#9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving
-------------------------------------+-------------------------------------
Reporter: simonpj | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.8.1
Component: Compiler (Type | Version: 7.8.2
checker) | Keywords: Roles,
Resolution: | QuantifiedConstraints
Operating System: Unknown/Multiple | Architecture:
| Unknown/Multiple
Type of failure: None/Unknown | Test Case: quantified-
| constraints/T9123{,a}
Blocked By: 15290 | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by simonpj):
> These would be easy to spot -- you just look for a type variable used as
an argument to another type variable
I'd say "possible" rather than "easy". You'd have something like
{{{
[W] m t1 ~R# m t2
}}}
and you want to say "one way to prove this would be to assume some
additional property of m (the quantified constraint) and, in addition,
prove `t1 ~R# t2`".
Wildly ad-hoc, but maybe. I wouldn't rush to do it.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:65>
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