Is Clojure object-oriented at its heart? (Polymorphism in seqs)

Question

Clojure is a functional lisp, reportedly not at all object-oriented, even though it runs on the JVM, a VM designed for an object oriented language. Clojure provides identical interfaces for iterating over lists and vectors by abstracting them to an interface called seq. This is even implemented internally using a Java interface called ISeq. Is this not an example of object-oriented abstraction? How can it be claimed that Clojure is not object-oriented?

I guess a corollary to this question--- when can polymorphism be considered distinct from object orientation?

Answer 1

Idiomatic Clojure favors defining independent functions that operate on a very small set of core data structures; this unbundling of methods and data is a strong statement against object orientation and in favour of a functional style. Rich Hickey (creator of Clojure) has repeatedly stated the importance of this; for example here: "Clojure eschews the traditional object-oriented approach of creating a new data type for each new situation, instead preferring to build a large library of functions on a small set of types.".

The reliance on the core data structures is even more important in Clojure than in other functional languages because you'll only reap the full benefits from Clojure's STM when you are using Clojure's persistent data structures.

I guess a corollary to this question--- when can polymorphism be considered distinct from object orientation?

I'm using Clojure's multimethods (i.e. polymorphic facilities) to dispatch to different implementations based on a filename's extension - not at all object oriented, but polymorphic.

Answer 2

I guess a corollary to this question--- when can polymorphism be considered distinct from object orientation?

Polymorphism has absolutely no relation to object-orientation. It simply means that the same operation can behave differently depending on the type(s) of its operands.

Functional languages like ML or Haskell have had polymorphism for more than 30 years, and someone with a better knowledge of PL history can probably point out some examples pre-1962 (i.e. pre-OO).

Christopher Strachey described the distinction between parametric polymorphism and ad-hoc polymorphism in 1967, so polymorphism must have already existed then. Since polymorphism was only introduced in OO in Simula-67, my guess is that polymorphism must have existed before it was introduced in OO.