Re: Scheme engraver structure questions

[Carl Sorensen]

On 12/26/11 12:50 PM, "David Kastrup" <address@hidden> wrote:

>Nicolas Sceaux <address@hidden> writes:
>
>> Le 9 févr. 2010 à 14:42, David Kastrup a écrit :
>>
>>> I find that syntax tiresome to read.  It requires evaluation and thus
>>> does not have a close correspondence to print syntax.
>>> 
>>> That's partly because of the way the Scheme engraver example has been
>>> written.
>>> 
>>> I'd much prefer it to use backquote syntax, such as
>>
>> A real improvement would be to add syntax to define scheme engravers,
>> something like:
>>
>> #(define-engraver example-engraver)
>>
>> #(define-engraver-method example-engraver initialize (trans)
>>    (display (list "initialize"
>>                   (ly:context-current-moment
>>                 (ly:translator-context trans)) "\n")))
>>
>> #(define-engraver-listener example-engraver rest-event (engraver event)
>>    (let* ((x (ly:engraver-make-grob engraver 'TextScript event)))
>>      (display (list "caught event" event "\ncreate:\n" x "\n"))
>>      (ly:grob-set-property! x 'text "hi")))
>>
>> \layout {
>>   \context {
>>     \Voice \consists #example-engraver
>>   }
>> }
>
>Actually, what about the following rather minimalistic approach?
>#(define-macro (make-engraver . translist)
>  (define (make-lambdas translist)
>   `(list
>     ,@(map (lambda (x)
>            (if (pair? (car x))
>             `(cons ',(caar x) (lambda ,(cdar x) ,@(cdr x)))
>             `(cons ',(car x) ,(make-lambdas (cdr x)))))
>       translist)))
>  (make-lambdas translist))
>
>\header {
>
>  texidoc = "\\consists can take a scheme alist as arguments, which
>  should be functions, which will be invoked as engraver functions."
>
>}
>
>\version "2.14.0"
>
>\layout {
>  \context {
>    \Voice
>    \consists
>    #(make-engraver
>      ((initialize trans)
>       (display (list "initialize"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n")))
>      ((start-translation-timestep trans)
>       (display (list "start-trans"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n")))
>      (listeners
>       ((rest-event engraver event)
>       (let*
>        ((x (ly:engraver-make-grob engraver 'TextScript event)))
>        (display (list "caught event" event "\ncreate:\n" x "\n"))
>        (ly:grob-set-property! x 'text "hi"))))
>      (acknowledgers
>       ((note-head-interface engraver grob source-engraver)
>       (display (list "saw head: " grob " coming from " source-engraver))))
>      (end-acknowledgers
>       ((beam-interface engraver grob source-engraver)
>       (display (list "saw end of beam: " grob " coming from "
>source-engraver))))
>      ((process-music trans)
>       (display (list "process-music"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n")))
>      ((process-acknowledged trans)
>       (display (list "process-acknowledged"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n")))
>      ((stop-translation-timestep trans)
>       (display (list "stop-trans"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n")))
>      ((finalize trans)
>       (display (list "finalize"
>                (ly:context-current-moment
>                 (ly:translator-context trans)) "\n"))))
>  }}
>
>\relative c' {
>  c8[ r c]
>}

Very nice!  The only think that I think ought to be added would be some
way of storing  a global status variable in the engraver, as they are
frequently used.  The status variable should be persistent between calls
to the engraver, I think.

Thanks,

Carl