Common Lisp中的类型类 [英] Typeclasses in Common Lisp

问题描述

我想知道是否有方法模拟Common Lisp中的Haskell的 typeclasses 。

$ b

通用函数允许重载，并且可以使用 deftype （例如，可以通过成员身份来定义一些实例列表）。

但是我不能在一个类型上 dispatch 。有没有办法让一个类的子类（和一个子类型）在其定义之后（例如，使 cons 类成为 sequence class，无需重新定义 cons ）？

谢谢。

解决方案

在Haskell中类型类是静态查找字典形式的接口实现的手段（类似于例如C ++中的vtables （但几乎）完全静态地使用，而不像在运行时动态调度的C ++）。然而Common Lisp是一种动态类型语言，所以这样的查找将毫无意义。但是，您可以在运行时自行查找类型实现（实例） - 这种设计在语言上不像Common Lisp那样难以想象。

如果您想要在动态设置中引用现有解决方案，Python的Zope具有非常类似特征的自适应机制。

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.

解决方案

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.

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