Dr. David J. Pearce

Whiley Puzzler

I was having an interesting discussion with a colleague today about various aspects of Whiley, and we came up with an interesting bit of example code which is something of a puzzler. Consider these two different versions of a function f(any):

Version 1:

int f(any x):
    if x is string:
        return g(x)
        return g(x)

Version 2:

int f(any x):
    return g(x)

The question is: are the above two examples equivalent or not? (answer below)

Well, the answer is: no, they’re not equivalent. You probably guessed that, but the question is: why not? Well, consider these two definitions for the function g() used in the examples:

int g(any x):
    return 1

int g(string x):
    return 2

In the first example, after the type test x is string Whiley’s flow-type system automatically retypes variable x to have type string. Therefore, the call g(x) on the true branch is statically dispatched to g(string), whilst on the false branch it dispatches to g(any).

Anyhow, this puzzler is an interesting (and perhaps surprising) artifact of the choice to employ flow typing in Whiley. I wonder what other artifacts we’ll find…