#7217: Unification of type variables in constraints
------------------------------+---------------------------------------------
Reporter: sjoerd_visscher | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1-rc1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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The following code works:
{{{
{-# LANGUAGE RankNTypes, ConstraintKinds, KindSignatures, GADTs #-}
import Data.Monoid
import GHC.Prim (Constraint)
data Dict :: Constraint -> * where
Dict :: a => Dict a
f :: (c (), c String) => (forall a. Dict (c a) -> a) -> ((), String)
f g = (g Dict, g Dict)
test :: ((), String)
test = f g
where
g :: Dict (Monoid a) -> a
g Dict = mempty
}}}
But this doesn't:
{{{
f :: (c (), c String) => (forall a. c a => a) -> ((), String)
f g = (g, g)
test :: ((), String)
test = f g
where
g :: Monoid a => a
g = mempty
}}}
With the errors:
{{{
Could not deduce (c0 String, c0 ()) arising from a use of `f'
Could not deduce (Monoid a) arising from a use of `g'
}}}
So it seems that type variables in constraints are not unified. At first I
thought this might not be easy to fix, but since there's a workaround I
guess it should be possible.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7217>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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