Re: Having trouble understanding optional and variable amount of arguments

[Simon Albrecht]

On 28.02.2018 23:54, Stefano Troncaro wrote:
> I need a little help again. I'm struggling to understand how one would 
> make a music function like \tempo, that accepts a variable number of 
> arguments. I couldn't find its definition so I couldn't look for myself.
>
> So far I know about making lambdas that take additional arguments and 
> store the extras in a list, but this does not work in a music function 
> (or if it works I don't know how to do it). And while I could make a 
> lambda that then passes the arguments to a music function, then I 
> can't use it with Lilypond syntax, so I'm forced to put the lambda and 
> all it's arguments between parenthesis.
>
> But \tempo doesn't require parenthesis, and the parser still 
> understands what is part of the function and what not. How would one 
> go about writing a function that behaves like this?

As has been said, \tempo isn’t a music function. However, you _can_ 
define functions with optional arguments; it’s mentioned in 
<http://lilypond.org/doc/v2.19/Documentation/extending/scheme-function-definitions>. 
The last optional argument has to be followed by a mandatory argument of 
different type, because that’s how the parser will recognise that 
optional arguments were skipped.

HTH, Simon