Are there problems that cannot be written using tail recursion?

Question

Tail recursion is an important performance optimisation stragegy in functional languages because it allows recursive calls to consume constant stack (rather than O(n)).

Are there any problems that simply cannot be written in a tail-recursive style, or is it always possible to convert a naively-recursive function into a tail-recursive one?

If so, one day might functional compilers and interpreters be intelligent enough to perform the conversion automatically?

Answer 1

Yes, actually you can take some code and convert every function call—and every return—into a tail call. What you end up with is called continuation-passing style, or CPS.

For example, here's a function containing two recursive calls:

(define (count-tree t)
  (if (pair? t)
    (+ (count-tree (car t)) (count-tree (cdr t)))
    1))

And here's how it would look if you converted this function to continuation-passing style:

(define (count-tree-cps t ctn)
  (if (pair? t)
    (count-tree-cps (car t)
                    (lambda (L) (count-tree-cps (cdr t)
                                                (lambda (R) (ctn (+ L R))))))
    (ctn 1)))

The extra argument, ctn, is a procedure which count-tree-cps tail-calls instead of returning. (sdcvvc's answer says that you can't do everything in O(1) space, and that is correct; here each continuation is a closure which takes up some memory.)

I didn't transform the calls to car or cdr or + into tail-calls. That could be done as well, but I assume those leaf calls would actually be inlined.

Now for the fun part. Chicken Scheme actually does this conversion on all code it compiles. Procedures compiled by Chicken never return. There's a classic paper explaining why Chicken Scheme does this, written in 1994 before Chicken was implemented: CONS should not cons its arguments, Part II: Cheney on the M.T.A.

Surprisingly enough, continuation-passing style is fairly common in JavaScript. You can use it to do long-running computation, avoiding the browser's "slow script" popup. And it's attractive for asynchronous APIs. jQuery.get (a simple wrapper around XMLHttpRequest) is clearly in continuation-passing style; the last argument is a function.