Apply values to a procedures

Questions & Answers
1

Hello,

I was in the need of applying a procedure to a result that return 3 values (x,y,z) to put this in a list.

I know this works:

(call-with-values (lambda () (values 1 2 3)) list)

but i find it not pleasant to wear the lamba () ... just because it is a convention? or perheaps there is something to understand that i do not see? so i decided to wrote this little macro in the way apply works:

(define-syntax apply-values
  (syntax-rules ()
    ((_ proc some-values) (call-with-values (lambda () some-values) proc))))

example:

(apply-values list (values 1 2 3))
'(1 2 3)

i think it is more simple and readable than the solution with call-with-values but i appreciate to know if i'm wrong or missing something and if it exist other syntax to do it?

Regards,

Damien

2

You can use (thunk body ...) instead of (lambda () ...) in this case.

It's not convention, though: it's explicitly used to delay evaluation. The call-with-values procedure expects a procedure of 0 arguments that produces multiple values, which are handed to the continuing procedure (second argument). Because Racket does not permit things like (add1 (values 1 2) 3), similarly (call-with-values (values 1 2 3) list) doesn't make sense.

The "extra notation" is not in the way so much with (call-with-values (thunk (f args)) list) (in other words, extracting the computation of the values to a named function; if it's nullary, simply (call-with-values f list)). Depending on your need you could also use define-values, let-values, match/values (and several other match cousins), or Qi.

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3

if it exist other syntax to do it?

Warning: Self-promotion ahead

SRFI 210: Procedures and Syntax for Multiple Values has a lot of useful stuff to make working with multiple values cleaner. It's available for Racket in my extra-srfi-libs package. With it, your example could be written as

$ racket
Welcome to Racket v8.15 [cs].
> (require srfi/210)
> (list/mv (values 1 2 3))
'(1 2 3)
>

Documentation for list/mv:

(list/mv element1 … producer)

Syntax: Each element and producer can be any expression.

Semantics: A list/mv expression is evaluated as follows: Each element and producer are evaluated in an unspecified order. A list constructed from the single values of the elements and the multiple values of producer in that order is then returned.

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