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?
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’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.