On Tue, Dec 05, 2006 at 11:08:07AM -0600, Creighton Hogg 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 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?
It's ugly but you could stick converters in the class itself: class MyClass a where isTypeA :: a -> Maybe A isTypeA _ = Nothing isTypeB :: a -> Maybe B isTypeB _ = Nothing isTypeC :: a -> Maybe C isTypeC _ = Nothing This limits you to a finite number of specific types, but I suspect that's unavoidable. Actually, your list can have any number of types in it, but you can only extract a fixed set of types. Another perhaps prettier way to do this would be to use an ADT to hold the elements of your list, so that the existentials could be reconstructed by pattern matching: data FunnyElement = ElemA A | ElemB B | ElemC C Again, it only works for lists containing a fixed set of types. -- David Roundy Department of Physics Oregon State University