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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.

Test your polymorphism knowledge.

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.