λ Tony's blog λ
The weblog of Tony Morris

Nothing returns anything, ever!

Posted on December 28, 2010, in Programming

I once had the following discussion with someone regarding a Java project:

Him: Once we did a project and we didn’t return anything from a method, ever (i.e. void) It went very well. Me: So everything was in CPS transform then so that you could simulate it? Him: No, we didn’t ever return anything ever! Me: So uh, it was uh, in CPS transform then, since er… you know… Him: No! Nothing. Return. Nothing. Ever. Void. Always! Me: Er yeah right uh huh.

This person operates under the illusion that I am in an intellectual battle with him, so I can excuse the somewhat awkward discussion. My efforts to destroy this destructive illusion have been repeated failure to date. That’s a side-story.

It occurred to me later, thanks to Edward Kmett, that perhaps it wasn’t a CPS transform but using exceptions instead! I doubt that was the case :) I’m also reasonably confident this discussion was argumentum ad ignorantiam – my converser doesn’t know what CPS transform means (so it can’t possibly be true!), but that’s cool – let’s not dwell on it. Instead, let’s have a look how you can simulate returning a value without actually returning a value using continuation-passing style (CPS)!

We may conjure up both simple and elaborate examples of functions that actually return a value:

Integer wibble(String s, double d)
Swizzler function1(int i, String s, Swazzle z)

I am going to use the trivial example (top one), but I hope you’ll be able to extrapolate to swizzlier cases.

First, note that Java methods may perform side-effects willy-nilly. Passing in a String and a double are not the only values that the function may access. The function may print to standard output, use variables in any scope, read/write files, the network and so on. This changes things a little (read: a lot) with regard to how we look at a function, so perhaps this is why this technique has apparent (read: apparent) merit.

Suppose a very simple interface:

interface To<a> {
  void to(A a);

Quite simply, this function takes a polymorphic (aka generic) value and performs a side-effect, perhaps using this value. It is important to point out that this polymorphic interface may be modelled differently e.g. it may not be polymorphic:

interface Stringer {
  void stringer(String s);

This interface is exactly equivalent to To<String> so I’m also trusting that you’ll extrapolate my generic example to other possibilities.

Now, instead of returning a type e.g. Integer in the wibble function, we may return void with an additional To<Integer> argument. We can do this for every single function that would otherwise return a value!

How does that work then? Simple really. We compute the Integer as we normally would in the body of wibble and instead of returning it (we can’t of course), we pass it to the to method of the given To<Integer> instance.

void wibble(String s, double d, To<integer> t)

And so on it goes, for every method.

Essentially, this results in a function that, instead of returning a value of type T, returns a value of the type (T => void) => void where (T => void) is represented by the To interface and is of course, in the position of a method argument where that method returns void. This is exactly equivalent to a specific example of a continuation. We are doing a CPS transform to simulate returning a value!

To summarise the formula:

For a method that would normally return a type T, we can transform it in such a way to accept an argument of type To<T> and returns void.

As for the merits of doing this, well I’ll leave that for another battle :) Further, there are some interesting properties about continuations, in particular, they are a monad, perhaps even the mother of all monads, but we’ll leave all that for another day too.

Edit: Per comments, I think a clarification is in order. Doing this is a very very bad idea on the JVM (without tail-call elimination – note that the IBM implementation only does direct TCE). Doing this results in a significant performance degradation and increased syntax for zero benefit. I didn’t bring this up because I was never going to get that far in the original conversation that I mention. Beware.