Re: Having trouble understanding optional and variable amount of arguments

[Urs Liska]

Am 1. März 2018 03:26:31 MEZ schrieb Stefano Troncaro <address@hidden>:
>Thank you! I see that this is not an option then. Also, I now
>understand
>why I couldn't make the optional arguments work, since I always left
>them
>for last.
>
>Do you know if it is possible to have a flexible amount of optional
>arguments that appear before the last mandatory one? Say, for example
>(define-music-function (arg1 args music) (number? ??? ly:music?) where
>arg1
>and music are mandatory, and basically everything between arg1 and the
>next
>music expression is compacted into a list and accessed as args in the
>body
>of the function. Not with that syntax necessarily, but something that
>allows for that kind of usage?

I *think* that's not doable in general (I'd love to be proven wrong), but if 
you can have all optional args as symbols you can use a dot-separated symbol 
list.

What I generally use is a \with {} argument which can take arbitrary named 
arguments. In oll-core there are a few helper functions for this, especially 
context-mod->props that turns the value into a regular alist.
I've just started thinking about an enhancement that
- allows to require some arguments
- can set missing args to defaults
- allows type checking

I'd be happy to collaborate on this.

Best
Urs
>
>2018-02-28 20:45 GMT-03:00 Simon Albrecht <address@hidden>:
>
>> 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/schem
>> e-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
>>