Constructing a java lambda expression?

Vibhu Mohindra vibhu.mohindra at
Wed Aug 24 02:58:50 UTC 2022

On 23/08/2022 05:27, Alan Ruttenberg wrote:
> There's a library I want to use that takes a lambda as an argument. 
> Anyone know how to construct one in ABCL?

I assume it's a Java library taking a Java Lambda that you want to call 
from ABCL.

A Java Lambda is really an instance of one of the classes in 
java.util.function. Which one depends on how many parameters it has and 
whether it returns a value. Let's assume your library wants a Java 
Lambda that has one parameter and returns a value. That's a 
java.util.function.Function. Say it looks like this:

public class Lib {
   public static void f(java.util.function.Function f) {
     System.out.println("You answered: " + f.apply(5));

such that it can be used from Java with a Java Lambda like this:

Lib.f((Object x) -> (Integer)x * (Integer)x);
=> You answered: 25

You'd like to give it a Lisp Lambda from ABCL as follows:

(jstatic "f" "Lib" #'(lambda (x) (* x x)))

but that's not allowed because although Java can implicitly convert from 
a Java Lambda to a java.util.function.Function, it can't convert from a 
Lisp Lambda to a java.util.function.Function.

A Lisp Lambda is really an org.armedbear.lisp.Function. So one solution 
is to adapt that.

import org.armedbear.lisp.*;
public class Adaptor implements java.util.function.Function {
   private org.armedbear.lisp.Function lispFn;
   public Adaptor(org.armedbear.lisp.Function lispFunction) {
     this.lispFn = lispFunction;
   public Object apply(Object input) {
     return lispFn.execute(
       JavaObject.getInstance(input, true)).javaInstance();

and use it from ABCL like this:

(jstatic "f" "Lib" (jnew "Adaptor" #'(lambda (x) (* x x))))
=> You answered: 25
=> NIL


I'm on Java 10, ABCL-1.4.0 (which is old).
I did this to build and run, starting with the jar and two java files in 
the current directory:
javac -classpath abcl-1.4.0.jar:.
java -classpath abcl-1.4.0.jar:. org.armedbear.lisp.Main
 > (jstatic "f" "Lib" (jnew "Adaptor" #'(lambda (x) (* x x))))

You can probably create the Adaptor class from within ABCL if you don't 
like that it's written in Java, but I don't remember how to. ABCL's 
documentation might describe such bytecode generation somewhere.

Java: java.util.function.*
ABCL: In org.armedbear.lisp, LispObject, JavaObject, Function


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