#6049: Kind variable generalization error in GADTs
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Reporter: goldfire | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.5
Keywords: PolyKinds | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: GHC rejects valid
program
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Comment(by simonpj):
Actually I think it's harder than I thought to "hack around". Suppose we
had
{{{
data SMaybe :: (k -> *) -> Maybe k -> * where
SNothing :: Int
-> (forall kk. forall (ss :: kk -> *). SMaybe ss Nothing)
-> SMaybe s a
}}}
I have used a nested forall, so now really impossible (without lots of
hackery) to make this work with a kind-monomorphic `SMaybe`.
This can't really happen unless we have explicit kind-foralls, but sooner
or later we will.
I think the simplest path is to say that for GADT-style syntax you must
give the kind of the type constructor in the header; and if you don't
you'll get `* -> * -> *`, ie not even higher kinds. This would mean that
some existing programs would need a kind signature.
{{{
data T a b where
MkT :: a b -> T a b
}}}
would need signatures, thus
{{{
data T (a : * -> *) b where
MkT :: a b -> T a b
or
data T :: (* -> *) -> * -> * where
MkT :: a b -> T a b
or
data T :: (k -> *) -> k -> * where
MkT :: a b -> T a b
}}}
But it would enforce good practice (put those kind signatures in! And I
suppose that this new behaviour could be enabled by `-XPolyKinds`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6049#comment:4>
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