List.java

public class List<T> implements Foldable<T>, Appendable<T, List<T>> {

  private final java.util.List<T> delegate;

  public List(java.util.List<T> delegate) {
    this.delegate = delegate;
  }

  public int size() {
    return this.delegate.size();
  }

  @Override
  public Appendable<T, List<T>> append(T t) {
    java.util.List<T> result = new ArrayList<>(delegate);
    result.add(t);
    return new List<>(result);
  }

  @Override
  public List<T> get() {
    return this;
  }

  @Override
  public <U> U foldRight(U seed, BiFunction<T, U, U> f) {
    U result = seed;
    ListIterator<T> li = delegate.listIterator(delegate.size());
    while(li.hasPrevious()) {
      result = f.apply(li.previous(), result);
    }
    return result;
  }

  @Override
  public <U> U foldLeft(U seed, BiFunction<U, T, U> f) {
    U result = seed;
    for (T t : delegate) {
      result = f.apply(result, t);
    }
    return result;
  }

  @Override
  public String toString() {
    return String.format("[%sNIL]", foldRight("", (a, b) -> String.format("%s, %s", a, b)));
  }

  @SafeVarargs
  public static <T> List<T> of(T... ts) {
    return new List<>(Arrays.asList(ts));
  }
}