Yet Another music macro proposal

[Erik Sandberg]

Hi,

I have yet another suggestion for how \relative can be soft-coded in a generic 
way. It's inspired by criticism of earlier attempts. It's similar, but not 
identical, to a previous suggestion.

Macros is that they operate on syntax, not on music. And the parser spits out 
music directly. So what I suggest, is to let the parser spit out yet another 
intermediate format, which is a Scheme expression that more or less 
corresponds to the the parse tree. So, for example,

\repeat volta 2 { c8 d e \foo }

would parse into a Scheme expression
(repeat 'volta 2 (sequential <c8> <d8> <e8> foo))
where <c8> etc. are Music objects (or possibly expressions that evaluate to 
Music objects).

When a top-level expression has been parsed into such scheme expression, it is 
evaluated in a Scheme module, where repeat, sequential etc. are defined as 
functions that return music; this evaluation produces the final music 
expression.

Some benefits:
- It will be easy to make \relative a music macro, by defining relative as a 
Scheme macro.
- The parser will be reduced to a thin layer of syntactic sugar, so a large 
portion of parser.yy can be moved to scheme. Application of music functions 
and dereferencing of variables is completely outsourced to guile.
- For those who want to do programming using lily syntax, it will be easy to 
export Scheme functions and macros to the lily parser at runtime. This will 
probably reduce the number of feature requests that come to our mailing 
lists.

-- 
Erik