Questions about using Scheme with Lilypond

[Tom Brennan]

I'd like to create a function that would allow me to create a `bookpart` from a list of arguments. E.g.,

```lilypond

\book {

\custom_bookpart "horn" \hn_movt_one \hn_movt_two

}

```

The reason is because I'd like to reuse header and page info, etc, without having to repeat those common variable names. I.e., replace this:

```lilypond

\book {

% clarinet

\bookpart {

\paper { ... }

\header {

...

instrument = \clarinet-name

}

\maestoso-clarinet-part

\scherzo-clarinet-part

}

% bassoon

\bookpart {

\paper { ... }

\header {

...

instrument = \bassoon-name

}

\maestoso-bassoon-part

\scherzo-bassoon-part

}

}

```

with this:

```lilypond

\book {

\part_bookpart \clarinet-name \maestoso-clarinet-part \scherzo-clarinet-part

\part_bookpart \horn-name \maestoso-hn-part \scherzo-hn-part

}

```

Is this kind of thing possible? I tried to create a music function, but I got hung up on a number of things:

1. You can't return a `bookpart` from a music function, and it looks like nothing happens when trying to return a `bookpart` from a scheme function/lambda, too:

```lilypond

custom_bookpart = #(lambda args #{ \bookpart {...} #})

\book {

#(custom_bookpart "horn" \hn_movt_one)

}

```

I guess I was under the impression that anything you can create in lilypond syntax can be created (also in some cases mutated?) in corresponding scheme code, but I'm having a hard time figuring out how to inspect the objects that result from various expressions (lilypond or scheme). E.g.,

```

custom_bookpart = #(lambda _ #{ \bookpart { ... } #})

#(display (custom_bookpart))

```

displays: #<Book>

Where do I find documentation about what a #<Book> is, and/or how do I inspect its properties, etc.? Can I mutate this (i.e., add/remove bookparts)? Why don't I get a #<BookPart> instead? It also seems like I should be able to use this function as an _expression_ in my book definition, but it doesn't work.

2. Applying variadic arguments (i.e., "splat"). E.g., in Clojure, you can do something like this:

```clojure

(defmacro foo [& body]

`(+ 1 ~@body))

(macroexpand '(foo 2 3 4))

=> (+ 1 2 3 4)

```

I tried using `custom_bookpart = #(lambda args ...)` like:

```lilypond

custom_bookpart = #(lambda args

#{

\bookpart {

\header { instrument = #(car args) }

#(cdr args)

}

#})

```

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?

Thanks

Tom