Question

What feature of what language does Clojure Protocols derive from?

Note: this is not a question about “How to Use Protocols”

This is a question about what inspired the idea.

In particular, what I want is to get something like:

• XYZ + ABC inspired Clojure Protocols

Then, I can go off and read about directions people went with XYZ and ABC for language features.

Thanks!

Answer 1

If you know Java, Clojure protocols are similar to Java interfaces – but better. The Clojure website says this about protocols (and there’s more background information and rationale about protocols on that page):

• 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

Furthermore, you might find Solving the Expression Problem with Clojure 1.2 an interesting read:

Clojure expert Stuart Sierra introduces you to new features in Clojure 1.2 that solve the Expression Problem, a classic programming dilemma. Protocols let you extend preexisting types to new methods, and datatypes let you extend preexisting methods to new types — all without changing the existing code. You’ll also see how Java™ interfaces and classes can interact with Clojure protocols and datatypes.