Java, the bad parts: Recursive lambdas

Have you ever tried to create a recursive lambda in Java?

You might ask why on earth would somebody do that. For you I have a confession to begin with: I've been spoiled. I've been spoiled with the overwhelming expressiveness of functional programming. I've been using Scala and JavaScript for more almost 2 years now, and that leaves a trace. I think about problems differently than an average Java developer does. Both Scala and JavaScript gives you the ability to express your problems more or less functionally. Neither of them are purely functional languages, but occupy a sweet spot to be fairly convenient to use. But why do then I mess around in Java? Not for fun, sure. I had a homework assignment for a university course which involved writing a breadth first traversal component for a graph. During traversal, you may inspect your current paths, and run some solver on it that classifies it as good or bad, and if it's bad you have to bail out with the level on which the bad path appeared and the path itself. It's pretty straightforward right? I have a stream of trees, which starts with an initial tree consisting only the root node, then I recursively flatMap the tree's outward edges to get a tree of longer paths, at every step I check if the new paths are bad... ... so I need lists, streams ... ... let's take a look at the standard Java collections.  

No. Just simply no.

So I started implementing my old own naive functional list. Guava provides a friendlier interface for Java collections, but I didn't want to involve additional dependencies, and also it would have solved my need for functional lazy streams. (This is only a guess, I haven't looked at Guava since 3 years.) Finally I'm at the point: creating my tree stream involved recursive lambdas. Let's take a look at a dummy example with natural numbers. In Scala it's as simple as a cake:

def naturals: Stream[Int] = 0 #:: naturals.map(_ + 1)

naturals.take(5).foreach(println)
// 0
// 1
// 2
// 3
// 4

In Java I tried this:

Function<Void, Stream<Integer>> n = (Void) -> Stream.cons(
   (Void _1) -> 0,
   (Void _1) -> n.andThen(x -> x.map(y -> y + 1)).apply(null));

Stream<Integer> naturals = n.apply(null);

naturals.take(5).foreach(x -> System.out.println(x));

This looks awful already, but even worse, it doesn't compile. Let's see why it looks awful:

• because it's Java?
• Java doesn't let you have a variable in the same name in an inner scope as in an outer one, while in Scala and JavaScript this works and called shadowing. That is why I had to find another name than x in my map function.
• I couldn't find a function type with no input arguments, I had to use Function<Void, T> instead. The Void class is a dummy class, that cannot be instantiated (null is the only valid value), so it acts like a void more or less.  That's why you will never need the value for a Void. So you can omit it right? Hold on. The same no redeclaration rule that we faced above holds for unnamed parameters. You cannot omit the name when an omitted name shadows another one omitted name. What the actual f*ck??? How can a non-existent name even shadow another? Someone really screwed something up with this language.

And then, it doesn't compile. It doesn't, because n may have been uninitialized at the time of use in the lambda. The compiler is thinking it's smarter than me. I understand it's a dangerous situation because if that lambda runs before defining the outer function, I get a NullPointerException. But I'm clever enough not to do this. Why doesn't it allow me? So I don't have to use this hack, which circumvents this foul attempt to stop me:

@SuppressWarnings("rawtypes")
final Function[] _n = { null };
@SuppressWarnings("unchecked")
Function<Void, Stream<Integer>> __n = _n[0];
__n = (Void) -> Stream.cons(
   (Void _0) -> 0,
   (Void _0) -> {
     @SuppressWarnings("unchecked")
     Stream<Integer> f = ((Function<Void, Stream<Integer>>)_n[0])
        .andThen(x -> x.map(y -> y + 1)).apply(null);
     return f;
   }
 );
_n[0] = __n;

Stream<Integer> naturals = __n.apply(null);

naturals.take(5).foreach(x -> System.out.println(x));
// 0
// 1
// 2
// 3
// 4

Beautiful. Luckily you can hide this ugliness if you abstract it:

static <U> Stream<U> recursively(Function<Void, U> base, Function<U, U> next) {
 @SuppressWarnings("rawtypes")
 final Function[] _n = { null };
 @SuppressWarnings("unchecked")
   Function<Void, Stream<U>> __n = _n[0];
   __n = (Void) -> Stream.cons(
     base,
     (Void _0) -> {
       @SuppressWarnings("unchecked")
       Stream<U> f = ((Function<Void, Stream<U>>)_n[0]).andThen(x -> x.map(y -> next.apply(y))).apply(null);
         return f;
       }
     );
   _n[0] = __n;
   return __n.apply(null);

The lesson was learned and it is: Java makes you life harder than it should be when you write functional code. Hard enough to don't even bother. Source for the List and Stream classes is here