[racket-dev] comments on "comments on learning Racket"

> From: Laurent <laurent.orseau at gmail.com>
> Subject: Re: [racket-dev] comments on "comments on learning Racket"
> Date: April 27, 2014 at 2:39:50 AM MST
> To: Matthias Felleisen <matthias at ccs.neu.edu>
> Cc: "dev at racket-lang.org Devs" <dev at racket-lang.org>
> 
> 
> On Sun, Apr 27, 2014 at 1:09 AM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
> ** tab completion must be faster
> 
> ** Do we need to add these to our library?
> 
> ;; (X ... -> Y ...) X *-> [List-of Y]
> (define (gather-return-values f . s)
>   (call-with-values (lambda () (apply f s)) list))
> 
> ;; Nat (X ... -> Y ...) X *-> Y
> (define (nth-return-value i f . s)
>   (call-with-values (lambda () (apply f s)) (lambda l (list-ref l i))))
> 
> I've been having a need of these functions on several occasions myself too.
> (Although the names are too long, but maybe just omitting `return` would be acceptable to my taste.)
>  

Hmmm… except for he name, I think I like that better than my own approach I wrote recently:

(require racket/function)

(define-syntax (values->list stx)
  (syntax-case stx ()
    [(_ (f args ...))
     #'(call-with-values (thunk (f args ...)) list)]))

(define-syntax (values-ref stx)
  (syntax-case stx ()
    [(_ (f args ...) n)
     (if (negative? (syntax->datum #'n))
         (raise-argument-error 'values-ref
                               "exact-nonnegative-integer?"
                               1
                               (syntax->datum #'(f args ...))
                               (syntax->datum #'n))
         #'(let ([lst (values->list (f args ...))])
             (if (<= 0 n (length lst))
                 (list-ref lst n)
                 (raise-arguments-error 'values-ref
                                    "index too large for values list"
                                    "index" n
                                    "in" lst))))]))

-Kevin
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