[Haskell-cafe] What is the rank of a polymorphic type?

[Stefan Holdermans]

Eugene,

> 1) Does there exist an authoritative source saying the same? Not that
> I'm doubting, just supposing that the source would have other
> interesting information, too :)

You may want to have a look at the already mentioned JFP-article by  
Peyton Jones et al. and perhaps the work of Kfoury and Wells.

> 2) Is it true that rank (forall a . a, forall a . a) == 0 ?

No, for pairs one takes the maximum of the constituent types. So, here  
you'd get rank 1.

Note that this is an impredicative type, which is yet another  
extension of the standard Hindley-Milner typing discipline.

Cheers,

   Stefan
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