To: "Creighton Hogg"
Hi Haskell-ers, So I think I understand the idea of creating a heterogenous list using typeclasses and existentials, but I don't see how to filter the list to retrieve elements of the list that are of only one type.
More concretely, taking the example herehttp://haskell.org/haskellwiki/Existential_typehow could we take a list of shapes [Shape] and pull out all objects that are Squares? I don't see an obvious way this makes sense. Is there a way of doing heterogenous lists that would make this possible?
Use filter, passing it a predicate that returns True for Squares and False otherwise: isASquare :: Shape -> Bool isASquare (Square _) = True isASquare _ = False filter isASquare myShapes Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/