Please explain lisp's multiple-value-bind

Question

I've read the docs (several different versions!) but I can't quite get my head wrapped around multiple-value-bind.

Here's what I (think I) know:

• The first parameter is a list of variables that are going to get bound.
• The next parameter is a list of values that get bound to the variables.
  • Am I right that these 2 lists have to be the same length?
• The last parameter (is it optional?) is a body of code that can act on the variables with their newly-bound values.

That sure seems to be how the docs read, and it fits with code I'm reading but not-quite following. I get into trouble when I try to create a multiple-value-bind statement from scratch, as a test. I end up with results like this:

? (mulitple-value-bind (x y z) (values 11 22 33) (+ x y z)) ;; EDIT: contains typo
> Error: Unbound variable: Y
> While executing: CCL::CHEAP-EVAL-IN-ENVIRONMENT, in process Listener(7).
> Type cmd-/ to continue, cmd-. to abort, cmd-\ for a list of available restarts.
> If continued: Retry getting the value of Y.
> Type :? for other options.
1 > 

(I was sort of hoping for output along the lines of 66.) (I'm using Clozure-CL if it matters, though I don't think it should.)

Also, I'm looking at some sample code (trying to understand Project Euler Problem 24) that reads like this:

(multiple-value-bind
    (q r)
    (floor n m)
    (cons (nth q lst) (permute-b r (remove-nth q lst)))
)

(NOTE: I may have mis-indented it, which may be affecting my lack of understanding)

What I don't get about this is it looks to me as if there are 2 variables being multiply-bound (q & r), but only one value (floor n m). Or is the other value the cons statement, and there is no body?!

As you can see, I completely don't get multiple-value-bind; please enlighten me.

Thanks!

Meyerbeer

Thanks Chris for pointing out typo in my original. I'm going to leave it in because otherwise the question doesn't make much sense (turns out I understood better than I thought; I just can't type! :) Excellent explanation, thanks! – Meyerbeer

"The next parameter is a list of values that get bound to the variables" -- no, it's a form that evaluates to multiple values "The last parameter (is it optional?)" -- it's an implicit progn, so 0 or more forms – Stash

Answer 1

Your first example with the "unbound variable" is due to your misspelling multiple-value-bind. Try fixing the spelling; you should see a different result.

As to your second question, floor returns two values, the floor and the remainder. Remember that values is not the only function that returns multiple values!

---

So, basically, the multiple-value-bind form looks like this:

(multiple-value-bind (var-1 .. var-n) expr
  body)

where expr is an expression that returns multiple values, which are bound to the variable names given in var-1 .. var-n; those variables are available for use in body. It is okay for expr to return more or fewer values than are given as variables; nil is used as the default value for any absent values, and any excess values are discarded.

Retrogressive

Aha! Sure enough, fixing typo fixed it. So maybe I understand m-v-b better than I thought I did! – Meyerbeer