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?

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. — glts
– glts, Commented Feb 8, 2016 at 12:57
@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? — onetom
– onetom, Commented Oct 24, 2022 at 2:24

Terje Dahl · Accepted Answer · 2016-02-08 11:58:34Z

10

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

answered Feb 8, 2016 at 11:58

Terje Dahl

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Comments

nha · Accepted Answer · 2018-10-27 13:31:56Z

7

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)))

edited Oct 27, 2018 at 13:31

nha

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answered Apr 4, 2017 at 0:08

Leon Barrett

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1 Comment

Jeremy Field Over a year ago

also see ask.clojure.org/index.php/767/…

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