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 data structures can only ever contain references to previous versions of themselves. Are there any Clojure APIs that could create a new data structure that has a reference to itself ?
Scheme has the letrec form which allows mutually recursive structures to be created - but, as far as I can tell, Clojure does not have anything similar.
This question is related to porting Clojure to iOS - which does not have garbage collection, but does have reference counting.
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.
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.
来源：https://stackoverflow.com/questions/3696214/is-it-possible-to-create-circular-references-in-clojure