Suppress the printing of the data an atom holds in the REPL? (or ref, agent, ...)

The following is perfectly valid Clojure code:

(def a (atom nil))
(def b (atom a))
(reset! a b)

it is even useful in situations where back references are needed.

However, it is annoying to work with such things in the REPL: the REPL will try to print the content of such references whenever you type a or b, and will, of course, generate a stack overflow error pretty quickly.

So is there any way to control/change the printing behaviour of atoms/refs/agents in Clojure? Some kind of cycle detection would be nice, but even the complete suppression of the deref'ed content would be really useful.

Solution

You can say

(remove-method print-method clojure.lang.IDeref)

to remove special handling of derefable objects (Atoms, Refs etc.) from print-method, causing them to be printed like so:

user=> (atom 3)
#<Atom clojure.lang.Atom@5a7baa77>

Alternatively, you could add a more specific method to suppress printing of contents of some particular reference type:

(defmethod print-method clojure.lang.Atom [a ^java.io.Writer w]
  (.write w (str "#<" a ">")))

user=> (atom 3)
#<clojure.lang.Atom@4194e059>