[racket] Style or and/define

Ah, very good, that's much better. Mystery solved.
(though I do kind of like the `and-let' with `#:let' keywords, since it
avoids the paren-verbosity of `let'.)

Thanks Клочков and Matthias,
Laurent


On Tue, Jun 11, 2013 at 8:44 PM, Matthias Felleisen <matthias at ccs.neu.edu>wrote:

>
> 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.orseau at 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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