How to print to STDERR in Clojure?

Question

The println function writes to STDOUT. How can we write messages to STDERR instead?

(println "Hello, STDOUT!") 

Answer 1

There is no specific function for this, however you can dynamically rebind the var holding the stream that println writes to like this:

(println "Hello, STDOUT!")  (binding [*out* *err*]   (println "Hello, STDERR!")) 

In my REPL, the colour indicates the stream (red is STDERR):

Answer 2

Plain Java methods work fine and will be understood easily by Clojure novices:

(.println *err* "Hello, STDERR!") 

Another alternative, just for fun, is clojure.java.io/copy:

(require '[clojure.java.io :as cjio])  (cjio/copy "Hello, STDERR!\n" *err*) (.flush *err*)