Instead of (a < SimpleListFilter) you can use a type-class constraint and an existential, or an explicit record of methods. In Haskell, anything you use will ba

Instead of (a < SimpleListFilter) you can use a type-class constraint and an existential, or an explicit record of methods. In Haskell, anything you use will basically be inferred, and Haskell can encode that, though you would have to choose different primitives to encode that relationship than subtypes.