Re: Questions about using Scheme with Lilypond

[Aaron Hill]

On 2020-11-15 12:03 pm, Tom Brennan wrote:
> I'd like to create a function that would allow me to create a 
> `bookpart`
> from a list of arguments. E.g.,
>
> [...]
>
> Is this kind of thing possible?

Yes-ish, see my post [1] last month about "returning" books and book 
parts.

[1]: 
https://lists.gnu.org/archive/html/lilypond-user/2020-10/msg00406.html


> 2. Applying variadic arguments (i.e., "splat"). E.g., in Clojure, you 
> can
> do something like this:
>
> [...]
>
> but you can't just throw a list in right there, right? It would need to 
> be
> expanded, in the `apply` sense. I assume Guile has macros, like 
> Clojure,
> but I don't know how to use them yet. Would that path lead me to 
> success
> here, though?

Guile does support macros, although these are not needed for procedures 
of variable arity.  Consider:

%%%%
\version "2.20.0"

#(define (foo . args) (format #t "\nargs=~s" args))
#(foo 1 2)
#(foo 'a 'b 'c)

#((lambda args (format #t "\nargs=~s" args)) 3 'd)
%%%%

====
Parsing...
args=(1 2)
args=(a b c)
args=(3 d)
====

The problem you are hitting is that music functions must have fixed 
arity.  There is limited support for optional arguments, but the parser 
ultimately needs to know how much input will be consumed by a function.  
Otherwise, you could invoke a music function and potentially read to the 
end of the file.

To work around this limitation, you can leverage existing constructs 
that contain variable numbers of things.  Consider a function that needs 
to accept one or more pitches.  You could instead accept ly:music? so 
multiple pitches are specified within curly braces (i.e. sequential 
music).  The music-pitches procedure will handle extracting the pitches 
from the provided music:

%%%%
\version "2.20.0"

baz = #(define-void-function (pitches) (ly:music?)
  (format #t "\npitches=~s" (music-pitches pitches)))
\baz a
\baz { b d f }
%%%%

====
Parsing...
pitches=(#<Pitch a >)
pitches=(#<Pitch b > #<Pitch d > #<Pitch f >)
====


-- Aaron Hill