Re: constructive criticism

[Nicolas Sceaux]

Wed, 7 Jan 2004 00:09:55 +0100, Han-Wen a dit : 

 > A similar idea has been on my mind for some time. In LilyPond 2.1 CVS,
 > you can now do

 >      \context Staff \with { StaffSymbol \set #'line-count = #4 }    { c4  }

 > to get a staff with 4 lines.

 > Currently, this only works for new contexts.

Indeed that would be great if \with could be used inside a existing
context:
  \context Staff {
    BLI
    \with { Stem \set #'thickness = #4 }
      { BLA }
    BLU
  }

 > It would be possible to add a hack that translates

 >      \context Staff \with { Stem \set #'thickness = #4 }
 >          { BLA }

 > to

 >      \context Staff {
 >            \property Staff.Stem \override #'thickness = #4
 >                      BLA
 >            \property Staff.Stem \revert #'thickness
 >      }

 > for existing Staff contexts.

[digression, don't have to be read]

That behaviour reminds me lisp macros.
Ie a special kind of "function" that can take a body of code as
argument, without evaluating it, in order to compute more elaborated
code. Ah, if only lilypond syntax were more lispy... adding such a
feature would be so trivial (in approximative guile scheme):

(defmacro* with-properties (context properties #:rest body)
  (let ((prop-vals (group properties 2)))
    `(sequential
       ,@(map (lambda (prop-val)
               `(context-property-override ,context 
                  ,(car prop-val)
                  ,(cadr prop-val)))
             prop-vals)
       ,@body
       ,@(map (lambda (prop-val)
               `(context-property-revert ,context 
                  ,prop-val))
             (map car prop-vals)))))

then:

(context Staff
  BLI
  (with-properties Staff (stem-thickness 4
                          slur-transparent #t)
    BLA)
  BLU)

== macro expansion of with-properties ==>

(context Staff
  BLI
  (sequential
    (context-property-override Staff stem-thickness 4)
    (context-property-override Staff slur-transparent #t)
    BLA
    (context-property-revert Staff stem-thickness)
    (context-property-revert Staff slur-transparent))
  BLU)

nicolas