#1123: Impredicativity bug: forall not hoisted properly
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Reporter: Ashley Yakeley <[EMAIL PROTECTED]> | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.6
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Comment (by guest):
the runA2 case was a simplification of the following code:
{{{
newtype M s a = M (StateT (UnionFind (RevertArray s) (STRef s) (ST s))
(ChoiceT (ST s)) a)
deriving (Monad, MonadPlus)
runM :: Int -> (forall s. M s b) -> [b]
runM n (M m) = runST (do uf <- newUnionFind n
findAll $ fmap fst $ runStateT uf m)
-- The following declarations are included to explain the types of the
functions in scope
newUnionFind :: PersistentArray a => Int -> ST s (UnionFind a s)
newUnionFind = undefined
findAll :: Monad m => ChoiceT m a -> m [a]
findAll = undefined
runStateT :: i -> StateT i m a -> m (a, i)
runStateT = undefined
}}}
My solution was to write an un-newtyping function ( M s a -> StateT
(UnionFind (RevertArray s) (STRef s) (ST s)) (ChoiceT (ST s)) a ) and call
that inside the "runST" but it wasn't clear why I would be required to do
so.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1123#comment:9>
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