Passing a Clojure function as java.util.Function

Question

As in topic, I'd like to use a Java method taking a Function as an argument and provide it with a Clojure function, be it anonymous or a regular one. Anyone has any idea how to do that?

• Marko Topolnik: (reify java.util.function.Function)?
• glts: The reify approach works but is overly verbose. I think we're going to see more and more Java APIs using the functional interfaces in java.util.function, so it would be good to fix this in Clojure itself. Clojure functions already implement Runnable and Callable.
• onetom: @glts are you aware of any discussion on the topic of extending the java.util.function.Function interface for java.lang.IFn or have java.lang.AFn implement it? It's still not done and I'm wondering why? github.com/clojure/clojure/blob/… Could it be that the Function also has a compose & andThen method, besides the main apply?

Answer 1

java.util.function.Function is an interface.
You need to implement the abstract method apply(T t).
Something like this should do it:

(defn hello [name]
 (str "Hello, " name "!"))
(defn my-function[]
 (reify 
 java.util.function.Function
 (apply [this arg]
 (hello arg))))
;; then do (my-function) where you need to pass in a Function

Answer 2

Terje's accepted answer is absolutely correct. But you can make it a little easier to use with a first-order function:

(defn ^java.util.function.Function as-function [f]
 (reify java.util.function.Function
 (apply [this arg] (f arg))))

or a macro:

(defmacro jfn [& args]
 `(as-function (fn ~@args)))

• Jeremy Field: also see ask.clojure.org/index.php/767/…