To supply callback function from clojure to java method expecting a lambda with interop

[Nagarajan N]

I started using clojure since last year. I like the language. It even made me more familiar with java. Before I detested java. Am moderately familiar with clojure way of making interop calls to java libraries. But I have no idea how to interop with a java library implementing java 8 lambdas as method params. I am not expecting any clean way to do this (I got that from this thread https://groups.google.com/forum/#!topic/clojure/WjTtqrLf_FY) . I just need a way (however verbose it is) to call a java method expecting a lambda.

For example a java 8 code like this

 

server.requestHandler(request -> {

  request.response().end("hello world!");

});

how do I call this in clojure ,something like this

 

(.requestHandler

   server 

   (some-magical-function-or-macro

     [request] 

     (doto request 

       (.response) 

       (.end "hello world!!")))) 

I 'd like someone to point me in the right direction for this.

[Gary Verhaegen]

The magical macro is reify. Ironically, you'll need to know the type of the lambda to use it from Clojure.

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[Nagarajan N]

I am really sorry. I am unclear about the type. Can you verify what I know is correct

I found an example java code which will consume lambda as an argument .

public class Main {

  interface MyCalcLambda {

    BigInteger run(BigInteger input);

  }

  public static void runCalc(MyCalcLambda calc) {

    // ???

  }

  public static void main(String[] args) {

    runCalc(a -> a.multiply(a));

  }

}

So here MyCalcLambda is the type of lambda. Is that correct?. So I can reify "MyCalcLambda" "run" 

method like any other java interface as long as I know the type "MyCalcLambda". Is that right? 

Library am going to use is an opensource library. I just need to write a layer in clojure so that It can be reused.So that shouldn't be big issue(I hope).

On Sunday, 20 September 2015 12:53:50 UTC+5:30, Gary Verhaegen wrote:

The magical macro is reify. Ironically, you'll need to know the type of the lambda to use it from Clojure.

On Sunday, 20 September 2015, Nagarajan N <onet...@gmail.com> wrote:

I started using clojure since last year. I like the language. It even made me more familiar with java. Before I detested java. Am moderately familiar with clojure way of making interop calls to java libraries. But I have no idea how to interop with a java library implementing java 8 lambdas as method params. I am not expecting any clean way to do this (I got that from this thread https://groups.google.com/forum/#!topic/clojure/WjTtqrLf_FY) . I just need a way (however verbose it is) to call a java method expecting a lambda.

For example a java 8 code like this

 

server.requestHandler(request -> {

  request.response().end("hello world!");

});

how do I call this in clojure ,something like this

 

(.requestHandler

   server 

   (some-magical-function-or-macro

     [request] 

     (doto request 

       (.response) 

       (.end "hello world!!")))) 

I 'd like someone to point me in the right direction for this.

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[ajoberstar]

Yes, your understanding is correct. MyCalcLambda is the one you'll want to reify, and it will be just like using reify on any interface.

After that thread you linked to, I did create ike.cljj (https://github.com/ike-tools/ike.cljj) which supports converting Clojure functions to lambdas.

For a lot of use cases, it's probably better to stick with reify, but ike.cljj can cut down on some of the boilerplate.

Andrew Oberstar 

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[Nagarajan]

Thank you everyone for clearing this up. Since my case is for a specific java library, it's source should give me an idea what to do. Java 8 callback is still possible with clojure. I really like to praise whoever responsible for making decision of java 8 lamdas as interface

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[Gary Verhaegen]

Note that you have to implement the one method of the functional interface. It may not always be called "run", as Java 8 places no restriction on the method name.

[Nagarajan]

Noted . Thank you very much