[Haskell-cafe] Rank-2 types and class constraints

[Ralf Laemmel]

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
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