#1807: type equality coercions not symmetric + order dependent
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Reporter: guest |
Owner: chak
Type: bug |
Status: new
Priority: normal |
Milestone: 6.10 branch
Component: Compiler (Type checker) |
Version: 6.8
Severity: normal |
Resolution:
Keywords: type equality coercions symmetry order constraints |
Difficulty: Unknown
Testcase: |
Architecture: Unknown
Os: Unknown |
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Comment (by Remi):
Not sure whether it's the same bug, but at least it looks very similiar.
In ghc-6.9.20071105, x typechecks, but y doesn't. And the only difference
is which side of the (a ~ b) the Eq constraint is on..
(there unfortunately are no newer binary linux i386 snapshots that
actually install..)
{{{
{-# LANGUAGE TypeFamilies, GADTs, KindSignatures, TypeOperators,
RankNTypes, FlexibleContexts #-}
module Foo where
data Equal a b = (b ~ a) => Refl
foo :: forall a b. ((a ~ b) => a -> b -> Bool) -> Equal a b -> a
-> b -> Bool
foo f Refl a b = f a b
x :: Eq b => Equal a b -> a -> b -> Bool
x = foo (==)
-- Doesn't typecheck
{-
y :: Eq a => Equal a b -> a -> b -> Bool
y = foo (==)
-}
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1807#comment:2>
GHC <http://www.haskell.org/ghc/>
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