Re: A few questions regarding markup

[Nicolas Sceaux]

Le 16 nov. 2009 à 20:32, David Kastrup a écrit :

> With very few exceptions (about 2 or 3, one being the harp-pedal code),
> all the commands appear to use the let-binding mechanism.

Indeed, when I introduced the property binding thing I changed consistently
all markup command definitions.  But it seems that I haven't documented
it properly.

Regarding the proper way to deal with the issue that you raise:

Currently, a define-builtin-markup-command expression like:

(define-builtin-markup-command (harp-pedal layout props definition-string) 
(string?)
  instrument-specific-markup
   ((size 1.0)
    (harp-pedal-details)
    (thickness 0.5))
   "doc"
   (make-harp-pedal layout props (harp-pedals-parse-string definition-string)))

expands into something like (among other things):

(define-public (make-harp-pedal-markup layout props definition-string)
   "doc"
   (let ((size (chain-assoc-get 'size props 1.0))
         (harp-pedal-details (chain-assoc-get 'harp-pedal-details props #f))
         (thickness (chain-assoc-get 'thickness props 0.5)))
     (make-harp-pedal layout props (harp-pedals-parse-string 
definition-string))))

In the case of the harp-pedal markup, a nice thing to have would be:

(define-public (make-harp-pedal-markup layout props definition-string)
   "doc"
   (let ((size (chain-assoc-get 'size props 1.0))
         (harp-pedal-details (chain-assoc-get 'harp-pedal-details props #f))
         (thickness (chain-assoc-get 'thickness props 0.5)))
     (let ((props (cons `((size . ,size)
                          (thickness . ,thickness))
                         props)))
       (make-harp-pedal layout props (harp-pedals-parse-string 
definition-string))))

That is, when a default value is provided for a property, the property
and its value (possibly the default one) are prepended to props, before
evaluating the command body, so that subsequent searches in props find the
default value set in the markup command.

As you point out, this introduces consing, and most of the time, this is 
useless,
as most of the commands do not call auxilary functions that in turn use the 
props
argument.

Moreover, I think that it may lead to confusion that the props argument is
bound to another value than the one that was passed to the function.

So imho, the way to handle it in the harp-pedal case is to explicitely prepend
property-value pairs to props before calling make-harp-pedal:

(define-builtin-markup-command (harp-pedal layout props definition-string) 
(string?)
   instrument-specific-markup
   ((size 1.0)
    (harp-pedal-details)
    (thickness 0.5))
   "Make a harp pedal diagram..."
   (make-harp-pedal layout
                    (cons `((size . ,size)
                            (thickness . ,thickness))
                            props)
                    (harp-pedals-parse-string definition-string)))

Nicolas