Is it possible to create circular references in Clojure?

Question

Ignoring native interop and transients, is it possible to create any data structures in Clojure that contain direct circular references ?

It would seem that immutable da

Answer 1

In Clojure most circular data structures will explicitely go through a ref type of some kind (eg atom).

However you can create a circular sequence (it's somewhat an oxymoron):

(let [a (atom nil)] (reset! a (lazy-seq (cons 1 @a))))

And since Clojure 1.2 with deftype you can create other datatypes which can introduce circularity without using explicitely (from the usercode at least) any kind of ref type.

Answer 2

You can create a circular reference very easily by putting some form of reference inside a data structure, then updating the reference to point back to the overall structure.

A trivial example:

(def a [(atom nil)])

(reset! (first a) a)

This will create a list with one element, which is an atom that points back at the list.