#6049: Kind variable generalization error in GADTs
----------------------------------------+-----------------------------------
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: |
----------------------------------------+-----------------------------------
Comment(by simonpj):
I was wrong in detail but I think right in principle. Consider
{{{
data Maybe a where
Nothing :: Maybe a
}}}
The two `a`'s have nothing to do with each other. We could equally well
write
{{{
data Maybe a where
Nothing :: Maybe b
or
data Maybe a where
Nothing :: forall b. Maybe b
or
data Maybe :: * -> * where
Nothing :: Maybe a
}}}
Now consider your exmaple:
{{{
data SMaybe :: (k -> *) -> Maybe k -> * where
SNothing :: forall (s :: k -> *). SMaybe s Nothing
}}}
The `k` in the `SNothing` is nothing to do with the `k` in the header,
just like the `Maybe` case above. So we could also write
{{{
data SMaybe :: (k -> *) -> Maybe k -> * where
SNothing :: forall (s :: kk -> *). SMaybe s Nothing
or
data SMaybe :: (k -> *) -> Maybe k -> * where
SNothing :: forall kk. forall (s :: kk -> *). SMaybe s Nothing
}}}
Actually we can't write the latter because we don't yet support explicit
kind foralls, but morally we could.
When kind-checking `SMaybe`, if we initially give it the ''monomorphic''
kind
{{{
SMaybe :: (kappa -> *) -> Maybe kappa -> *
}}}
where `kappa` is a kind unification variable, then if we kind-check the
type of `SNothing` we'll have a problem because `SMaybe` is insufficiently
polymorphic to kind-check the type
{{{
forall kk. forall (s::kk -> *). SMaybe s Nothing
}}}
We could hack our way round this, perhpas by (in effect) not kind-
genrealising the type of `SNothing`. But it seems smelly. I'd be
interested in what Stephanie and Dimitrios think.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6049#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
