:raising_hand:

Welcome @mark.piffer :grinning:

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Which is the correct way to letor define (i.e. bind names to) the first n values of a list? I tried to recurse with a values but this is obviously the wrong approach (define (values-rec n lst) (if (= n 1) (values lst) (values (first lst) (values-rec (- n 1) (rest lst)))))

Usage hould have been (values-rec 3 '("a" "b" "c" "d" "e"") -> "a" "b" '("c" "d" "e")

For (define-values ...)of course, I forgot

If you have a list of values, say, '(1 2 3) and want to compute (f 1 2 3) you can use apply as in (apply f '(1 2 3)). In this case, you want to compute (values 1 2 3), so you can use (apply values '(1 2 3)).

Where ’(1 2 3) needs to be a list of the first n elements of lst.

I would probably look at split-at to split the list, and then define-values with apply values on the head; or, use match-define with a pattern for your specific n (maybe a macro could generate part of it?)

I’m having a conceptual problem in understanding what values is actually returning

it is not a list, that much I do understand

Normally an expressions evaluates to a single value: > (+ 1 2) 3

Some computations naturally have two results.

yes, I read that in the Racket Reference

For example: > (split-at ’(a b c d) 2) needs to return both (a b) and (c d).

but what is the type of a two-value result?

You can think of two values as an implicit tuple.

If you think of the computation in term of a stack, (values 10 11) simply pushes two values to the stack.

In (define-values (a b) (values 10 11)) the identifiers are bound to the two top values on the stack.

how do functions consume these sstacked arguments?

only via the bound names?

Most contexts expect to receive only one value. So for example (+ (values 10 11) 3) gives an error.

You know about define-values and let-values but besides them call-with-values is the most imported way of using multiple values.

> (https://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._call-with-values%29%29\|call-with-values (https://docs.racket-lang.org/reference/lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29\|lambda () (https://docs.racket-lang.org/reference/values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29\|values 1 2)) https://docs.racket-lang.org/reference/generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29\|+) 3

ah ok! That was the missing piece. Interesting.

thank you

I don’t think multiple return values are unique to Scheme/Racket, but most programming languages don’t have them. Most other languages use tuples instead.

I believe compose can also be an alternative to call-with-values. I think ((compose list split-at) '(a b c d) 2) will give a first-class list instead of two values.

just tried it and it does :)

Also, let-values , define-values, etc. are handy. <file:///Users/badkins/sync/github/racket/racket/doc/reference/values.html#%28def.%28%28quote.~23~25kernel%29._values%29%29|See here>

Conversely, compose1 forces a single value, which can be useful