[Haskell-cafe] Aren't type system extensions fun?
zunino at di.unipi.it
Thu May 29 07:44:24 EDT 2008
Kim-Ee Yeoh wrote:
> How about
> foo :: (exists. m :: * -> *. forall a. a -> m a) -> (m Char, m Bool)
Thank you: I had actually thought about something like that.
First, the exists above should actually span over the whole type, so it
becomes a forall because (->) is contravariant on its 1st argument:
foo :: forall m :: * -> * . (forall a. a -> m a) -> (m Char, m Bool)
This seems to be Haskell+extensions, but note that m above is meant to
be an arbitrary type-level function, and not a type constructor (in
general). So, I am not surprised if undecidability issues arise in type
Another valid type for foo can be done AFAICS with intersection types:
foo :: (Char -> a /\ Bool -> b) -> (a,b)
But I can not comment about their inference, or usefulness in practice.
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