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<http://haskell.org/haskellwiki/Existential_type>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?
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