ポリモーフィズムのためのプロトコルとマルチメソッド

Clojure separates data from behavior. Protocols and multimethods let you dispatch behavior without classes or inheritance.

A defprotocol names a set of method signatures, like an interface.

Use defrecord to create a type that implements the protocol.

Protocols dispatch on the type of the first argument. This is fast and the most common polymorphism in Clojure.

Use extend-protocol to add behavior to types you do not own, even built-in ones.

Protocols only dispatch on type. When you must dispatch on a value, multiple arguments, or a computed key, reach for multimethods.

defmulti takes a dispatch function; defmethod supplies an implementation per dispatch value.

Provide a :default method to handle unmatched dispatch values gracefully.

The dispatch function can compute any value, even from multiple arguments, giving multimethods far more flexibility than protocols.

Prefer protocols for type-based, performance-sensitive dispatch. Use multimethods when dispatch logic is richer than a single type.

Both tools are open: new types or methods can be added later in separate namespaces without editing the original code, solving the expression problem cleanly.

You learned protocols with defrecord and extend-protocol for type dispatch, multimethods with defmulti/defmethod for value dispatch, default methods, and how both keep code open for extension.