Description Moritz Eysholdt 2014-03-26 06:35:48 EDT

newArrayList and newHashSet are quite convenient. It would be nice if there would be a utility function that can convert a self-implemented linked list into an Iterable or Iterator.

I'm aware this scenario can easily be implemented using while-loop or a 'tradition' for-loop. However, both force me into imperative programming. Sometimes it's nice to stay functional.

Example:

----------------- Main.xtend -------------------
import static extension IterableLiterals.*

class Main {
	def static void main(String[] args) {
		val item = new MyLinkedListItem => [
			name = "first"
			next = new MyLinkedListItem => [
				name = "second"
				next = new MyLinkedListItem => [
					name = "third"
					next = new MyLinkedListItem => [
						name = "fourth"
					]
				]
			]
		]

		for (x : item.newIterable[i|i.next])
			println("imperative:" + x.name)

		println(item.newIterable[next].map["functional:" + name].join("\n"))
	}

}

class MyLinkedListItem {
	@Property String name
	@Property MyLinkedListItem next
}

----------------------------------------------


----------------- stdout ---------------------
imperative:first
imperative:second
imperative:third
imperative:fourth
functional:first
functional:second
functional:third
functional:fourth
----------------------------------------------


------------------ IterableExtensions.java -------------
import java.util.Iterator;

import com.google.common.base.Function;

public class IterableLiterals {
	/**
	 * Returns an Iterable that will return <i> head </i> as its first 
	 * item and use the function <i> trail </i> to compute all follow-up items.
	 * 
	 * @param head
	 * @param trail
	 * @return
	 */
	public static <T> Iterable<T> newIterable(final T head, final Function<T, T> tail) {
		return new Iterable<T>() {
			@Override
			public Iterator<T> iterator() {
				return IteratorLiterals.newIterator(head, tail);
			}
		};
	}
}
------------------------------------------------------


---------------------- IteratorExtensions.java --------------
import java.util.Iterator;

import com.google.common.base.Function;

public class IteratorLiterals {
	/**
	 * Returns an Iterator that will return <i> head </i> as its first item 
	 * and use the function <i> trail </i> to compute all follow-up items.
	 * 
	 * @param head
	 * @param trail
	 * @return
	 */
	public static <T> Iterator<T> newIterator(final T head, final Function<T, T> tail) {
		return new Iterator<T>() {

			private T next = head;

			@Override
			public boolean hasNext() {
				return next != null;
			}

			@Override
			public T next() {
				T oldNext = this.next;
				this.next = tail.apply(this.next);
				return oldNext;
			}

			@Override
			public void remove() {
				throw new UnsupportedOperationException();
			}
		};
	}

}
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