Reusing Lambdas from Java in Clojure

[Rohan Bryant]

I am running an application on JRE8 JVM. The application defines a sizeable number of lambdas.
I am also augmenting the functionality with Clojure based reporting functions.

It would be ideal for me to simply reuse the lambdas defined in my Java8 source as closures in
Clojure.

How do I accomplish this? If I cannot do this, I would have do unfortunately redefine the lambdas as
Clojure closures.

Regards,

/Kobe

[Francis Avila]

I'm not sure what you mean by "as closures in Clojure". Java lambdas capture their scope lexically just like a Clojure function would, so if you define something in Java it's going to capture the surrounding Java scope. I'm not sure what it would mean for Java code to capture a Clojure scope! You can of course call Java code that uses lambdas from Clojure, or even call lambdas directly (although it may be trickier than normal because of the invisible interfaces involved). Could you expand more on what you would like to do, maybe with a code example?

There is also a discussion about Java 8 lambda interop in Clojure which may be of help to you.

[Sean Corfield]

Rohan, like Francis I'm not entirely sure what you're trying to do, without seeing some code, but I wonder if this article will help you get a feel for how you might call Java closures from Clojure (by understanding how they compile):

http://www.infoq.com/articles/Java-8-Lambdas-A-Peek-Under-the-Hood

[Peer Reynders]

Rohan Bryant wrote:
It would be ideal for me to simply reuse the lambdas defined in my Java8 source as closures in Clojure.

Could you please elaborate what you mean by as closures in Clojure?
Have you already tried something and run into problems?
How does your use case differ from basic reuse as illustrated in the following example?

// mylambdas/Lambda.java
package mylambdas;

import java.util.function.BiFunction;

public class  Lambda {
  public static BiFunction<String, String, Integer> fs = (s1,s2) -> {
    if (s1.length() < s2.length()) {
      return -1;
    } else if (s1.length() > s2.length()) {
      return 1;
    } else {
      return 0;
    }
  };

  public BiFunction<String, String, Integer> fi = (s1,s2) -> fs.apply(s1,s2);
}

; src/driver/core.clj
(ns driver.core
  [:import mylambdas.Lambda])

(defn -main [] 
  (println 
    (.apply Lambda/fs "one" "three")) ; "fs" is a static class property
  (->                                 ; thread first macro
    (Lambda.)                         ; create Lambda instance
    .fi                               ; get "fi" from the instance
    (.apply "three" "one")            ; apply parameters to execute "fi"
    println))                         ; print the result

$ javac mylambdas/Lambda.java $ jar cf mylambdas.jar ./mylambdas/Lambda.class $ java -cp clojure-1.8.0.jar:mylambdas.jar clojure.main -i src/driver/core.clj -m driver.core -1 1 $