Monday, September 29, 2008

JavaScript completion value: pros and cons

In JavaScript the completion value is the result value of a computation, which may result even from evaluating a statement. Simple statements like
2 + 2;
have result values (in this case 4), but so do complex statements like
for (var i = 0; i < n; i++)
Notice that if n is less than [edit: or equal to] 0 in this example, there won't be any computation at all, in which case the statement can't really produce any reasonable value.

I've just figured out how to articulate an intuition I've long felt about the completion value. The great thing is that it mitigates some of the awfulness of statements: even imperative code can still produce values! This means you can have some of the syntactic convenience of statements for writing effectful code, while still producing results. But the thing that always made me uncomfortable was how code like the for-loop above screws with the idea of a tail call. Because the for-loop may or may not produce a value, there's a sort of implicit return happening after the loop.

There might be tricks you could pull using something like continuation marks to allow tail position inside of loops, but I think a simpler answer in a JavaScript with proper tail calls would be to say that loops do not preserve tail position. So essentially tail position would include the last statement in a block, the arms of an if statement, and the arms of a conditional expression, and that's pretty much it. Inside a try-block, a loop, or a switch (because of the reliance on break) there simply wouldn't be any tail positions.

Note that a) JavaScript doesn't have tail calls, and b) the only places where you can even observe the completion value of a statement are at a REPL or with eval. But I think JavaScript could have proper tail calls (potentially), and I've always wondered whether the completion value should remain a hack isolated to eval or actually see its way into the design of other features for JavaScript.

No comments: