[GHC] #8634: Relax functional dependency coherence check ("liberal coverage condition")
GHC
ghc-devs at haskell.org
Thu May 28 20:17:38 UTC 2015
#8634: Relax functional dependency coherence check ("liberal coverage condition")
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Reporter: danilo2 | Owner:
Type: feature request | Status: new
Priority: high | Milestone: 7.12.1
Component: Compiler | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: None/Unknown | Unknown/Multiple
Blocked By: | Test Case:
Related Tickets: #1241, #2247, | Blocking:
#8356, #9103, #9227 | Differential Revisions: Phab:D69
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Comment (by atnnn):
What is the status of this feature?
I ran into another case where it might be useful:
{{{#!hs
{-# LANGUAGE FunctionalDependencies, FlexibleInstances,
UndecidableInstances, ConstraintKinds, GADTs #-}
class F a | -> a
instance (eq ~ (a ~ ()), eq) => F a
}}}
GHC considers this illegal, but in practice it satisfies the functional
dependency.
--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634#comment:59>
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