danieldietrich

import static java.lang.System.*;

import java.util.function.BiFunction;
import java.util.function.Function;

// Implementation of a pseudo-GADT in Java, translating the examples from
// http://www.cs.ox.ac.uk/ralf.hinze/publications/With.pdf
// The technique presented below is, in fact, just an encoding of a normal Algebraic Data Type
// using a variation of the visitor pattern + the application of the Yoneda lemma to make it
// isomorphic to the targeted 'GADT'.

danieldietrich

sealed trait TreeLike[M[_]] {
  def node[A](f: M[A], g: M[A]): M[A]
}

sealed trait Tree[+A] { self =>
  def flatMap[B](f: A => Tree[B]): Tree[B] = self match {
    case Leaf(a) => f(a)
    case Node(l, r) => Node(l.flatMap(f), r.flatMap(f))
  }
}

danieldietrich

interface Monad<T> {
    <U> Monad<U> flatMap(Function<? super T, ? extends Monad<U>> mapper);
}

interface Tree<T> extends Monad<T> {
    @Override
    <U> Tree<U> flatMap(Function<? super T, ? extends Tree<U>> mapper);
}

interface BinaryTree<T> extends Tree<T> {

danieldietrich

/*     / \____  _    ______   _____ / \____   ____  _____
 *    /  \__  \/ \  / \__  \ /  __//  \__  \ /    \/ __  \   Javaslang
 *  _/  // _\  \  \/  / _\  \\_  \/  // _\  \  /\  \__/  /   Copyright 2014-2015 Daniel Dietrich
 * /___/ \_____/\____/\_____/____/\___\_____/_/  \_/____/    Licensed under the Apache License, Version 2.0
 */
package javaslang.control;

import javaslang.Function1;

import java.lang.invoke.MethodType;

danieldietrich

    static class JavaslangGoodies {

        static <T, R> MethodType getType(SerializableFunction<T, R> function) throws Exception {
            // FIXED, DON'T CHECK FOR CAPTURED LAMBDA PARAMETERS
            return getLambdaSignature(function);
        }

        static SerializedLambda getSerializedLambda(Serializable lambda) throws Exception {
            final Method method = lambda.getClass().getDeclaredMethod("writeReplace");
            method.setAccessible(true);

danieldietrich

class BinaryTree<T> extends Tree<T> {

    // ...

    default boolean isBinarySearchTree() {

        class MinMax<U extends Comparable<Object>> {

            final U min;
            final U max;

danieldietrich

/**
 * This is a Java class using Javaslang's Lazy
 */
class LazyDemo {{
    
    // -- Lazy instance
    
    Lazy<String> value = Lazy.of(() -> "Yay!");
    
    value.toString(); // = Lazy(?)

danieldietrich

As stated by Oracle,

You can serialize a lambda expression if its target type and its captured arguments are serializable. However, like inner classes, the serialization of lambda expressions is strongly discouraged.

Interesting that the following works:

import java.util.Optional;

public class Test {

danieldietrich

If you are willing to use Javaslang, Java 8 will look more like Scala.

Scala

val filteredList =
  stringsToFilterList.filter(s => s.nonEmpty && s.startsWith("somePredicate"))

Javaslang

danieldietrich

public static void main(final String... args) {
    // this is javaslang.collection.Stream
    Stream.of(0, 1, 2, 3, 13, 14, null, -1)
            .peek(n -> out.print(format("%d -> ", n)))
            .map(Match.as(Object.class) // match as function
                    .when(Objects::isNull).then("!")
                    .whenIs(0).then(nil())
                    .when(is(1)).then("one")
                    .whenIs(13).then(() -> "unlucky")
                    .whenIs(14).then(printIt())