I generally use the Typeable class for this. In that example, you'd want:
class Typeable a => Shape_ a instead of just class Shape_ a and then your filter predicate would look like: isSquare :: Shape -> Bool isSquare (Shape s) = typeOf s == typeOf square where square :: Square ; square = undefined (warning: I didn't try this code.) That adds a little overhead (particularly in that everything must now derive Typeable) but is one of the better solutions I've seen. /g On 12/5/06, Creighton Hogg <[EMAIL PROTECTED]> wrote:
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 here how 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? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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