Re: Scheme engraver structure questions

[David Kastrup]

Nicolas Sceaux <address@hidden> writes:

> Le 9 févr. 2010 à 21:32, David Kastrup a écrit :
>
>> If you really must, you should probably use make-procedure-with-setter
>> in order to be able to use
>> 
>> (define (engraver-method engraver 'initialize) ...
>
> According to:
>
>   
> <http://www.gnu.org/software/guile/manual/html_node/Adding-or-Setting-Alist-Entries.html>
>
> "The only safe way to update an association list variable when adding or
> replacing an entry like this is to set! the variable to the returned value"
>
> You cannot just assoc-set! an associative list.  So a procedure with setter
> does not help in that case (unless I've misunderstood something).

Well, there are several ways to address this problem.

But the most obvious one is that a Scheme engraver may contain an
association list, and currently may be specified as one in a list of
engravers, but should itself be a recognizable data structure, even if
it just means that its associations start with a known car, like
'(engraver (initialize . ...
or
'((type . engraver) (...

One reason is that it has to be identifiable by ly:translator?.  But
personally, I'd make it a separate data structure internally containing
either the association list, or a number of fields that are initialized
from such an association list.  _If_ we want to support the current
syntactic entry of anonymous engravers.  Which I don't necessarily
consider a good idea.  It might be preferable for an anonymous form to
look like

(make-engraver #:initialize (lambda () ...) #:finalize ...)

since that saves the programmer the bother of messing with quoting.

-- 
David Kastrup