Typeclasses in Common Lisp

Question

I'm wondering if there's a way to emulate Haskell's typeclasses in Common Lisp.

Generic functions allow overloading, and it's possible to define types using deftype(which could be defined by membership to some list of instances, for example).

But I can't dispatch on a type. Is there a way to make a class a subclass(and a subtype) of some other class after its definition(e.g. making the cons class a subclass of a sequence class, without redefining cons)?

Thanks.

Answer 1

Type classes in Haskell are a means to statically look up implementations for "interfaces" in the form of dictionaries (similarly to how vtables in e.g. C++ are used but (almost) fully statically, unlike C++ which does dynamic dispatch at runtime). Common Lisp however is a dynamically typed language so such lookup would make no sense. However you can implement your own look up of "type class" implementations (instances) at runtime — a design not too hard to imagine in a language as expressive as Common Lisp.

P.S. Python's Zope has an adaption mechanism with very similar charactetistics, if you feel like referring to an existing solution in a dynamic setting.