How to call a Clojure variadic function from Java

Question

Is it possible to declare a variadic function in Clojure that can be called as a varargs method from Java?

Consider this extract from some code under development:

(ns com.mydomain.expression.base 
  (:gen-class
    :name com.mydomain.expression.Base
    :methods [^:static [exprFactory [String String ?????] Object]]
  )

(defn expr-factory
  ; worker function that is also called from Clojure
  [id func & args]
  (let [ex ("construction code here")]
    ex))

(defn -exprFactory
  ; interface method called from Java
  [idStr funcStr & argsArray]
  (apply expr-factory idStr funcStr (seq argsArray)))

Is there anything I can put in place of ????? to allow Java to call the exprFactory method and to know that it is a varargs method:

import com.mydomain.expression.Base;
...
Object e1 = Base.exprFactory("e1", "day");
Object e2 = Base.exprFactory("e2", "length", "string");
Object e3 = Base.exprFactory("e3", "*", 4, 5);
Object sum = Base.exprFactory("sum", "+", e1, e2, e3);

To make this a little clearer, I know I use Object in place of ????? and change exprFactory to:

(defn -exprFactory
  ; interface method called from Java
  [idStr funcStr argsArray]
  (apply expr-factory idStr funcStr (seq argsArray)))

..but that means I have to write Java calls like this:

import com.mydomain.expression.Base;
...
Object e1 = Base.exprFactory("e1", "day", new Object[0] ));
Object e2 = Base.exprFactory("e2", "length", new Object[] { "string" }));
Object e3 = Base.exprFactory("e3", "*", new Integer[] { 4, 5 }));
Object sum = Base.exprFactory("sum", "+", new Object[] { e1, e2, e3 }));

Again, I know I could write a varargs wrapper method in Java that calls the non-variadic exprFactory, but I'd like to avoid that if possible.

@andrew That was the first thing I tried, and I also tried Object... which would match the Java syntax. — Dave Hartnoll
– Dave Hartnoll, Commented May 2, 2012 at 12:52
since varargs are just a cheat by the compiler that just look like an array in the type signature, you should be able to specify a String[] with this: "[Ljava.lang.String;" instead of ?????. I tried that, it compiles, but eclipse won't recognized the varargs, it insists on a String[] even tho a java class and a gen-class class have identical type sigs. You might have more luck — sw1nn
– sw1nn, Commented May 2, 2012 at 14:51
I don't believe this is currently possible. I posted my investigation to the clojure ML - groups.google.com/forum/?fromgroups#!topic/clojure/HMpMavh0WxA — sw1nn
– sw1nn, Commented May 3, 2012 at 9:34
@sw1nn Thanks for following this up further. I'd got as far as using your trick to force the Java side to provide the array but not with the preferred variadic syntax. It would be nice to have a clean way to do this from Clojure - perhaps :methods [^:static [exprFactory [String String & Object] Object]] and have Clojure automatically handle the interpretation of the array as a sequence. — Dave Hartnoll
– Dave Hartnoll, Commented May 3, 2012 at 11:20

mikera · Accepted Answer · 2012-05-03 04:03:34Z

1

I would suggest writing your helper function on the Java side, something like the "applyToHelper" in clojure.lang.AFn

This would take the form of a Java function that looks something like:

   public Object invokeVariadic(IFn function, Object... args) {
     switch (args.length) {
       case 0:
         return function.invoke();
       case 1:
         return function.invoke(args[0]);
       /// 20-odd more cases
     }
   }

It's a bit of a hack and depends on the internal definition of clojure.lang.IFn, but at least you will get relatively nice variadic syntax on the Java side (i.e. no need to do the new Object[] {...} stuff).

answered May 3, 2012 at 4:03

mikera

107k28 gold badges265 silver badges427 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Dave Hartnoll Over a year ago

I think that might be overkill for what I'm trying to achieve. For clarity, on the Clojure side I want to ultimately call expr-factory with an optional sequence of arguments (operands for an expression) - a ref to that sequence is stored in the returned Object. The (apply ...) form in the -exprFactory function is only used to expand the Java array. Perhaps there's a more efficient way to interface -exprFactory and expr-factory as I'm aware the args currently go from Java array -> sequence -> flattened arg list -> sequence.

Collectives™ on Stack Overflow

How to call a Clojure variadic function from Java

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related