Rank-2 types and class constraints
Hi, Just out of curiosity (I cannot come up with a practical example): Why doesn't the following piece of code type check in GHC (with extensions)?
foo :: (forall a . (Eq a) => a) -> Integer foo = undefined
It seems like the type-class constraint is playing a decisive rôle here, since the following does check.
bar :: (forall a . a) -> Integer bar = undefined
TIA, Stefan
Hi, foo _ = undefined works fine. Otherwise the poor little "a" has no chance to get disambiguated. ... Ambiguous type variable `a' in the top-level constraint ... Ralf Stefan Holdermans wrote:
Hi,
Just out of curiosity (I cannot come up with a practical example): Why doesn't the following piece of code type check in GHC (with extensions)?
foo :: (forall a . (Eq a) => a) -> Integer foo = undefined
It seems like the type-class constraint is playing a decisive rôle here, since the following does check.
bar :: (forall a . a) -> Integer bar = undefined
TIA,
Stefan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-caf e
participants (2)
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Ralf Laemmel -
Stefan Holdermans