[racket] Style or and/define

From: Клочков Роман (kalimehtar at mail.ru)
Date: Tue Jun 11 13:39:52 EDT 2013

 You may use srfi/2  http://docs.racket-lang.org/srfi-std/srfi-2.html

(require srfi/2)
(define (get-x-spot char-width)
    [dc (get-dc)]
    [style (or (send (get-style-list) find-named-style "Standard")
                       (send (get-style-list) find-named-style "Basic"))])
     (let*-values ( [(fnt) (send style get-font)]
                  [(xw _1 _2 _3) (send dc get-text-extent "x" fnt)])
           (+ left-padding (* xw char-width))))))

Вторник, 11 июня 2013, 18:49 +02:00 от Laurent <laurent.orseau at gmail.com>:
>When I see what Robby is forced to write when following the Style:
>I cannot help but think that something is wrong somewhere (it may not be the Style, and in case it wasn't clear I'm certainly not criticizing Robby's code).
>Using `let' and `and' instead, although being a bit better since it avoids all the [else #f], is not that big an improvement:
>(define (get-x-spot char-width)
>  (and
>   char-width 
>   (let ([dc (get-dc)])
>     (and
>      dc
>      (let ([style (or (send (get-style-list) find-named-style "Standard")
>                       (send (get-style-list) find-named-style "Basic"))])
>        (and
>         style
>         (let*-values ([(fnt) (send style get-font)]
>                       [(xw _1 _2 _3) (send dc get-text-extent "x" fnt)])
>           (+ left-padding (* xw char-width)))))))))
>Actually I think here the right thing to do might be to allow for internal definitions inside `and':
>(define (get-x-spot char-width)
>  (and char-width
>       (define dc (get-dc))
>       dc
>       (define style (or (send (get-style-list) find-named-style "Standard")
>                         (send (get-style-list) find-named-style "Basic")))
>       style
>       (define fnt (send style get-font))
>       (define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt))
>       (+ left-padding (* xw char-width))))
>Isn't it *much* more readable? (shorter, avoid rightward drift, less parens, vertical alignment)
>Since it's not the first time I find the need for such internal definitions in `and', maybe this is something to consider for future addition to Racket? Or have some people already identified some problems with this idea?
>I've played a bit with it if you want to try by your own:
>(not sure I got it all good with syntax-parse though)
>  Racket Users list:
>   http://lists.racket-lang.org/users

Роман Клочков
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