For play.libs.F ?

gistfile1.java

package scrapbook;

import java.lang.reflect.InvocationTargetException;

public class Test {

    public static void main(String[] args) {

        // Standard function
        Function<String,String> toUpperCase = new Function<String,String>() {{ b = a.toUpperCase(); }};

        assertTrue( toUpperCase.invoke("kiki").equals("KIKI") );

        // Closure
        final Integer maxLength = 6;
        Function<String,Boolean> isLengthOk = new Function<String, Boolean>() {{ b = a.length() <= maxLength; }};

        assertTrue( isLengthOk.invoke("coco") == true );
        assertTrue( isLengthOk.invoke("guillaume") == false );

    }

    // ----- With:

    public static void assertTrue(Boolean v) {
        if(!v) throw new RuntimeException("Assertion failed");
    }

    public static class Val<T> {

        T t;

        public Val(T t) {
            this.t = t;
        }

    }

    public static abstract class Function<A, B> {

        A a;
        B b;
        
        private Function() {
            try {
                if (input.get() != null) {
                    a = ((Val<A>) input.get()).t;
                } else {
                    _closedValues = new Object[this.getClass().getDeclaredFields().length];
                    for (int i = 0; i < _closedValues.length; i++) {
                        _closedValues[i] = this.getClass().getDeclaredFields()[i].get(this);
                    }
                    a = (A)"";
                }
            } catch (Exception e) {
                throw new RuntimeException("Function declaration", e);
            }
        }
        
        private Object[] _closedValues;
        static final ThreadLocal<Val> input = new ThreadLocal<Val>();

        public B invoke(A a) {
            input.set(new Val<A>(a));
            try {
                Function<A, B> invoked = (Function<A, B>) this.getClass().getDeclaredConstructors()[0].newInstance(_closedValues);
                return invoked.b;
            } catch (InvocationTargetException e) {
                throw new RuntimeException("Function invocation exception", e.getTargetException());
            } catch (Exception e) {
                throw new RuntimeException("Function instantiation exception", e);
            } finally {
                input.remove();
            }
        }
        
    }


}