Protocols were introduced in Clojure 1.2
Protocols were introduced in Clojure 1.2.
Clojure is written in terms of abstractions. There are abstractions for sequences, collections, callability, etc. In addition, Clojure supplies many implementations of these abstractions. The abstractions are specified by host interfaces, and the implementations by host classes. While this was sufficient for bootstrapping the language, it left Clojure without similar abstraction and low-level implementation facilities. The protocols and datatypes features add powerful and flexible mechanisms for abstraction and data structure definition with no compromises vs the facilities of the host platform.
There are several motivations for protocols:
- Provide a high-performance, dynamic polymorphism construct as an alternative to interfaces
- Support the best parts of interfaces
° specification only, no implementation
° a single type can implement multiple protocols - While avoiding some of the drawbacks
° Which interfaces are implemented is a design-time choice of the type author, cannot be extended later (although interface injection might eventually address this)
° implementing an interface creates an isa/instanceof type relationship and hierarchy - Avoid the 'expression problem' by allowing independent extension of the set of types, protocols, and implementations of protocols on types, by different parties
° do so without wrappers/adapters - Support the 90% case of multimethods (single dispatch on type) while providing higher-level abstraction/organization
Reference: http://clojure.org/protocols
