CODEWITHSUNDEEP
haskellcommon-lisptypeclass
Charles Langlois
Charles Langlois

Reputation: 4298

Typeclasses in Common Lisp

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.

Upvotes: 8

Views: 1019

Answers (2)

jch
jch

Reputation: 5651

You cannot modify the class hierarchy in the way you envision, but you can achieve pretty much the same effect.

Suppose that your definition of a sequence is that it has a method for the function sequence-length.

(defclass sequence ...)
(defmethod sequence-length ((s sequence)) ...)

Then you can easily extend your sequence-length method to conses:

(defmethod sequence-length ((s cons))
    (length s))

Did that create a new class that includes cons? Not really. You can express the type of things that have a sequence-length method by saying (or sequence cons), but that's not really useful.

Upvotes: -1

Erik Kaplun
Erik Kaplun

Reputation: 38217

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.

Upvotes: 2

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