What does IFn in Clojure stand for?

Question

In the Java source code for Clojure, I have seen:

import clojure.lang.RT;
import clojure.lang.IFn;

RT seems to stand for "run-time" (although no official source is given in the answer: In the clojure source code, what does RT stand for?)

But what does IFn stand for?

Answer 1

IFn means Interface Function. Clojure uses the capital "I" prefix for Java interfaces and "A" for Java abstract classes.

See also IFn Javadoc and IFn source

Answer 2

in clojure's terms there is also one more difference between fn and ifn:

user> (defn f [x] x)
#'user/f

user> (map fn? [f #(list %) :keyword 'sym {:a 10} #{123} [1 2 3] 10 "asd"])
(true true false false false false false false false)

user> (map ifn? [f #(list %) :keyword 'sym {:a 10} #{123} [1 2 3] 10 "asd"])
(true true true true true true true false false)

so, as you can see fn? is true for actual functions only, while ifn? if true for anything, that could be called as a function (like keywords, maps e.t.c)