16 Feb
2009
16 Feb
'09
1:22 p.m.
Am Montag, 16. Februar 2009 19:04 schrieb Kim-Ee Yeoh:
Despite its rank-2 type, runST really doesn't have anything to do with existential quantification.
First, I thought so too but I changed my mind. To my knowledge a type (forall a. T[a]) -> T' is equivalent to the type exists a. (T[a] -> T'). It’s the same as in predicate logic – Curry-Howard in action. However, if we talk about existential types in Haskell, we usually mean these special algebraic data types whose declarations have a forall part before a data constructor. So it’s better to talk about rank-2 (or rank-n) polymorphism when talking about runST. Best wishes, Wolfgang