#5771: Confusing printout with PolyKinds
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Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler | Version: 7.4.1-rc1
Keywords: PolyKinds | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: Other
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Changes (by anders_):
* cc: andy@… (added)
Comment:
Also, the kind parameter for a type counts as a parameter for whether a
class
involving that type needs MultiParamTypeClasses or not. Unless it’s *,
strangely.
That wasn’t a very clear explanation, so here’s an example. Involving a
variant
monad class *and* HList, naturally. :)
{{{
> {-# LANGUAGE DataKinds, PolyKinds, GADTs, TypeOperators #-}
> {- # LANGUAGE MultiParamTypeClasses #-}
>
> class IndexedMonad m where
> unit :: a -> m i i a
> bind :: m i j a -> (a -> m j k b) -> m i k b
>
> newtype IndexedIO i j a = IndexedIO {runIndexedIO :: IO a}
>
> -- i and j are both *; instance is accepted
> instance IndexedMonad IndexedIO where
> unit = IndexedIO . return
> bind m k = IndexedIO $ runIndexedIO m >>= runIndexedIO . k
> infixl 1 `bind`
>
> data HList xs where
> N :: HList '[]
> (:>) :: a -> HList as -> HList (a ': as)
> infixr 5 :>
>
> newtype HLState xs ys a = HLState {runHLState :: HList xs -> (a, HList
ys)}
>
> -- i and j are now [*]; rejected with the MPTCs message
> instance IndexedMonad HLState where
> unit x = HLState $ \s -> (x, s)
> bind (HLState f) k = HLState $ \xs ->
> case f xs of (a, ys) -> runHLState (k a) ys
}}}
(I ''think'' this is another manifestation of the same bug; if not, I can
submit a new ticket.)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5771#comment:3>
GHC <http://www.haskell.org/ghc/>
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