Nice, I wish Oleg had also defined let*-and, which is what I think you really
want here:
(define-syntax let*-and
(syntax-rules ()
[(_ ((x:id e:expr) (y:id f:expr) ...) body0:def-or-expr body1 ...)
(let ((x:id e:expr))
(and x:id (let*-and ((y:id f:expr) ...) body0:def-or-expr body1 ...)))]
[(_ ((x:id) (y:id f:expr) ...) body0:def-or-expr body1 ...)
(let ((x:id x:id))
(and x:id (let*-and ((y:id f:expr) ...) body0:def-or-expr body1 ...)))]
[(_ () body0:def-or-expr body1 ...)
(let ()
body0:def-or-expr
body1 ...)]))
;; ----------------------------------------------------------------
(class object%
(super-new)
(define/private (get-x-spot.v2 char-width)
(let*-and ([char-width]
[dc (get-dc)]
[style (or (send (get-style-list) find-named-style
"Standard")
(send (get-style-list) find-named-style
"Basic"))])
(define-values (fnt) (send style get-font))
(define-values (xw _1 _2 _3) (send dc get-text-extent "x" fnt))
(+ left-padding (* xw char-width)))))
-- Matthias
On Jun 11, 2013, at 1:39 PM, Клочков Роман wrote:
> You may use srfi/2 http://docs.racket-lang.org/srfi-std/srfi-2.html
>
> (require srfi/2)
> (define (get-x-spot char-width)
> (and-let*
> ([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.ors...@gmail.com>:
> When I see what Robby is forced to write when following the Style:
> https://github.com/plt/racket/commit/09d636c54573522449a6591c805b38f72b6f7da8#L4R963
>
> 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:
> https://gist.github.com/Metaxal/5758394
>
> (not sure I got it all good with syntax-parse though)
>
> Laurent
>
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>
>
>
> --
> Роман Клочков
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