Re: limitation in make-engraver?

[David Kastrup]

Thomas Morley <address@hidden> writes:

> Hi,
>
> it looks like the make-engraver-makro fails to accept certain variables.
>
> In the code belowe 3 engravers are defined using different syntax. All
> three try to use a predefined variable.
> The first, defined with make-engravers fails.
>
>
> \version "2.19.52"
>
> #(define ev 'note-event)
>
> #(define test-engr
>   (lambda (context)
>     (make-engraver
>       (listeners
>         (
>          (ev engraver event)
>          ;(note-event engraver event)

ev is the same as 'note-event, not note-event.

make-engraver is a macro.  It (obviously) does not evaluate a lot of the
structure but converts it into something else.  Creating such a macro in
your own manner will most of the time require using, well, a macro
(which gets unexpanded arguments and expands its result).  Or you use a
function but call primitive-eval on its result.

>            (write ev)
>            (format #t "\n\nnote-event found from test-engraver\n"))))))
>
> #(define test-I-engr
>   (lambda (context)
>     `((listeners
>         (
>          ,ev
>          ;note-event
>           .
>           ,(lambda (engraver event)
>              (write ev)
>              (format #t "\n\nnote-event found from test-I-engraver\n")))))))
>
> #(define test-II-engr
>   (lambda (context)
>     (list
>       (cons 'listeners
>         (list
>           (cons
>             ev
>             ;'note-event
>             (lambda (engraver event)
>                (write ev)
>                (format #t "\n\nnote-event found from 
> test-II-engraver\n"))))))))
> \layout {
>   \context {
>     \Voice
>     \consists #test-engr
>     %\consists #test-I-engr
>     %\consists #test-II-engr
>   }
> }
>
> {
>   c'4
> }
>
> Am I missing something or is it a limitation of make-engraver?

Like define , some of its "argument" contains symbols that are never
evaluated but used directly.  You cannot deliver them with an expression
unless you add a separate explicit expansion step instead of relying on
normal Scheme parsing for evaluating the macro.

-- 
David Kastrup