The Seasoned Schemer, letcc and guile

Question

A few questions here, regarding letcc that is used in The Seasoned Schemer.

(define (intersect-all sets)
  (letcc hop
    (letrec
      ((A (lambda (sets)
            (cond
              ((null? (car sets)) (hop '())
              ((null? (cdr sets)) (car sets))
              (else
                (intersect (car sets)
                           (A (cdr sets)))))))
       ; definition of intersect removed for brevity
      (cond
        ((null? sets) '())
        (else (A sets))))))

1. I think I understand what letcc achieves, and that is basically something like catch and throw in ruby (and seemingly CL), which basically means a whole block of code can be cut short by calling whatever the named letcc is. This feels like the least "functional" thing I've come across in this short series of books and it makes me feel a bit hesitant to use it, as I want to learn a good functional style. Am I just misunderstanding letcc, or is it not really a functional programming concept and only exists to improve performance? The whole idea that I can be in the middle of some routine and then suddenly get to another point in the code feels a bit wrong... like abusing try/catch in Java for program flow.

2. letcc doesn't seem to exist in the version of guile (1.8.7) I have installed in OS X. Is there another name for it that I should be looking for in guile?

3. If I'm misunderstanding letcc by comparing it with try/catch in Java, or catch/throw in ruby (which is not exception handling, just to be clear, for the non-rubyists), how exactly does it work, at the functional level? Can it be expressed in a longer, more complex way, that convinces me it is functional after all?

Answer 1

1. "Functional" has several meanings, but no popular meaning contradicts continuations in any way. But they can be abused into creating code that is hard to read. They're not tools that can be "abused for program flow" -- they are program flow tools.

2. Can't help you there. I know that there was semi-recent on continuations in Guile, but I don't know where things stand. It should definitely have call-with-current-continuation, usually also under the more friendly name of call/cc, and let/cc is a simple macro that can be built with call/cc.

   I can tell you that in Racket there is a let/cc builtin together with a bunch of others builtins in the same family, and additionally there's a whole library of various control operators (with an extensive list of references.).

3. Simple uses of let/cc are indeed similar to a catch/throw kind of thing -- more specifically, such continuations are commonly known as "escape continuations" (or sometimes "upward"). This is the kind of use that you have in that code, and that is often used to implement an abort or a return.

   But continuations in Scheme are things that can be used in any place. For a very simple example that shows this difference, try this:

   (define (foo f) (f 100))
   (let/cc k (+ (foo k) "junk") (more junk))
   

4. Finally, if you want to read more about continuations, you can see the relevant parts of PLAI, there's also a more brief by-example overview that Matthew Might wrote, and you can see some class notes that I wrote that are based on PLAI, with some examples that were inspired by the latter post.