A lambda is just an interface implementation.

On Tue, 23 Aug 2022 at 05:40, Alan Ruttenberg alanruttenberg@gmail.com wrote:

There's a library I want to use that takes a lambda as an argument. Anyone know how to construct one in ABCL?

Thanks, Alan

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:

//Lib.java 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.

//Adaptor.java 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

---- Notes:

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:. Adaptor.java Lib.java 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.

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

On 24/08/2022 09:42, Alessio Stalla wrote:

Function is a convenient interface for cases when a more specific interface does not exist. But any Java interface with a single abstract method can be the target of a lambda expression:

[...] Lambda is just syntax sugar for interfaces with a single method, [...].

You're absolutely right. I'd forgotten all this.

Vibhu

So in order to implement this I need to know the interface to use for the lambda? Presumably I can determine this with reflection.

An example of one of the functions that takes the lambda is defined:

public final NitfSegmentsFlow forEachImageSegment(final Consumer<ImageSegment> consumer)

But consumer has two methods: accept and andThen

Eventually it looks like accept is called on the consumer. So in this case it looks like I need to use jinterface-implementation and define the accept method, no lambda necessary?

What confuses me now is if a lambda is passed as the test code I'm reading does:

forEachImageSegment(imageSegment -> {do something})

How does java know that the lambda is the implementation of accept rather than andThen. Or more practically, how do I figure out which method to implement without reading the source code?

Thanks, Alan

On Wed, Aug 24, 2022 at 4:14 AM Alessio Stalla alessiostalla@gmail.com wrote:

Function is a convenient interface for cases when a more specific interface does not exist. But any Java interface with a single abstract method can be the target of a lambda expression:

new Thread(() -> { System.out.println("foo"); }).run();

The Thread constructor takes a Runnable argument, not a Function.

Lambda is just syntax sugar for interfaces with a single method, so you can reproduce them on ABCL with the jinterface thing + some macrology.

On Wed, 24 Aug 2022 at 05:10, Vibhu Mohindra vibhu.mohindra@gmail.com wrote:

...

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:

//Lib.java 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.

//Adaptor.java 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

---

Notes:

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:. Adaptor.java Lib.java 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.

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

-- Vibhu

Thanks.

(defun get-abstract-methods (classname) (map 'list #"getName" (remove-if-not (lambda(x) (let ((modifiers (#"getModifiers" x))) (and (#"isAbstract" 'reflect.Modifier modifiers) (not (#"isStatic" 'reflect.Modifier modifiers))))) (#"getDeclaredMethods" (find-java-class classname)))))

(defun java-lambda-parameter-information (classname methodname position) (let* ((method (find methodname (#"getDeclaredMethods" (find-java-class classname)) :key #"getName" :test 'equalp)) (parameter-type (elt (#"getParameterTypes" method) position)) (abstract-methods (get-abstract-methods parameter-type))) (assert (#"isInterface" parameter-type) () "lambda parameter type ~a should be an interface but isn't" (#"getName" parameter-type)) (assert (= (length abstract-methods) 1) () "Parameter ~a type ~a has more than one abstract method: ~{~a~^, ~}" position (#"getName" parameter-type) abstract-methods) (list (#"getName" parameter-type) (car abstract-methods))))

(java-lambda-parameter-information 'Nitfsegmentsflowimpl "forEachImagesegment" 0) -> ("java.util.function.Consumer" "accept")

Cheers, Alan

On Thu, Aug 25, 2022 at 6:35 AM Alessio Stalla alessiostalla@gmail.com wrote:

Because *andThen *is a *default method *in the interface, i.e. it's not abstract: https://docs.oracle.com/javase/8/docs/api/java/util/function/Consumer.html So, effectively, the interface only has one abstract method, and that's the target of lambda conversion. Now I have no idea if and how ABCL deals with default methods. If it's still stuck on Java 5/6 compatibility then I suspect that it can't, at least not in its Java code.

On Thu, 25 Aug 2022 at 06:37, Alan Ruttenberg alanruttenberg@gmail.com wrote:

...

So in order to implement this I need to know the interface to use for the lambda? Presumably I can determine this with reflection.

An example of one of the functions that takes the lambda is defined:

public final NitfSegmentsFlow forEachImageSegment(final Consumer<ImageSegment> consumer)

But consumer has two methods: accept and andThen

Eventually it looks like accept is called on the consumer. So in this case it looks like I need to use jinterface-implementation and define the accept method, no lambda necessary?

What confuses me now is if a lambda is passed as the test code I'm reading does:

forEachImageSegment(imageSegment -> {do something})

How does java know that the lambda is the implementation of accept rather than andThen. Or more practically, how do I figure out which method to implement without reading the source code?

Thanks, Alan

On Wed, Aug 24, 2022 at 4:14 AM Alessio Stalla alessiostalla@gmail.com wrote:

...

Function is a convenient interface for cases when a more specific interface does not exist. But any Java interface with a single abstract method can be the target of a lambda expression:

new Thread(() -> { System.out.println("foo"); }).run();

The Thread constructor takes a Runnable argument, not a Function.

Lambda is just syntax sugar for interfaces with a single method, so you can reproduce them on ABCL with the jinterface thing + some macrology.

On Wed, 24 Aug 2022 at 05:10, Vibhu Mohindra vibhu.mohindra@gmail.com wrote:

...

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:

//Lib.java 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.

//Adaptor.java 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

---

Notes:

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:. Adaptor.java Lib.java 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.

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

-- Vibhu

Hi Vibhu,

I think our emails crossed. Fortunately I don't have to ask the list each time :-) I have a successful POC using the functions I defined in my last email and one more.

(defun java-lambda-interface (class-or-object methodname position fn) (let ((classname (cond ((symbolp class-or-object) class-or-object) ((jinstance-of-p class-or-object (find-java-class 'class)) (#"getName" class-or-object)) (t (#"getName" (jobject-class class-or-object)))))) (uiop/utility:call-with-muffled-conditions ;; so we don't get warnings about stubs (lambda() (destructuring-bind (interface methodname) (java-lambda-parameter-information classname methodname position) (jinterface-implementation interface methodname fn))) '(simple-condition))))

I can then use this to create the interface and pass it where a lambda is being used as such:

(let ((jlambda (java-lambda-interface doc "forEachImageSegment" 0 #'print))) (#"forEachImageSegment" doc jlambda))

and it works.

I'll do some work to integrate into jss and avoid doing all of this dynamically every time the lambda is used, where possible etc.

Thanks to you both! Alan

On Thu, Aug 25, 2022 at 10:19 AM Vibhu Mohindra vibhu.mohindra@gmail.com wrote:

Hi Alan,

On 25/08/2022 06:36, Alan Ruttenberg wrote:

...

So in order to implement this I need to know the interface to use for the lambda? Presumably I can determine this with reflection.

You learn this not by reflection but by reading the library's source code, which you've done below to determine that the interface is Consumer not Function.

...

An example of one of the functions that takes the lambda is defined:

public final NitfSegmentsFlow forEachImageSegment(final Consumer<ImageSegment> consumer)

But consumer has two methods: accept and andThen

As Alessio said andThen() has a default implementation in Consumer (just as it did in Function).

...

Eventually it looks like accept is called on the consumer.

That sounds right.

...

So in this case it looks like I need to use jinterface-implementation and define the accept method, no lambda necessary?

You need to implement an accept() method somehow. One way is similar to how I'd shown an implementation of java.util.function.Function.apply(). (At the time I didn't know what type of object your library wanted so I assumed for concreteness that it was Function. Now we know it's Consumer. The principle of the technique I showed still applies.)

I showed how to implement an adaptor class in Java. I noted that you could probably do the equivalent from ABCL. Alessio pointed out that that way was to use jinterface-implementation. So you don't _need_ to use a jinterface-implementation approach, but you can.

You're right, it doesn't seem like you need to think about Java lambdas, just a Java interface (Consumer) and getting a new class that implements that interface and instantiating it to get an object. Whether you write that class in Java or by using jinterface-implementation in ABCL is a different matter.

...

What confuses me now is if a lambda is passed as the test code I'm reading does:

forEachImageSegment(imageSegment -> {do something})

How does java know that the lambda is the implementation of accept rather than andThen.

As Alessio described, Java can only know this if an interface has a single (abstract) method in it. It then assumes that the lambda represents an implementation of that single method. You can see from the javadoc for Consumer that though it has more than one method, all but accept() have a "default" next to them, which means that the Consumer interface already provides a concrete implementation of them. That is, they aren't abstract. accept() doesn't have a "default" next to it, so it's abstract, meaning all concrete classes that claim to implement the interface must provide a concrete implementation of (at least) accept().

...

Or more practically, how do I figure out which method to implement without reading the source code?

1. By reading the javadoc for Consumer. I showed you an example for

Function, which specifies a number of methods but only one (apply()) was abstract. https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html https://docs.oracle.com/javase/8/docs/api/java/util/function/Consumer.html

2. By asking on this list apparently :-) The answer is accept().

---

If you want to take a small step first, modify what I sent you earlier to work with Consumer instead of Function. The modification should be trivial. Once you've got that working, and if you don't like that the adaptor is written in Java or like experimenting, then explore the jinterface-implementation approach to do everything directly from ABCL.

-- Vibhu