Re: Optional arguments

[David Kastrup]

Paul Morris <address@hidden> writes:

> David Kastrup wrote
>> The last argument of a function is never "really" optional (the only way
>> to get the default value is by writing \default).
>> 
>>> Relevant docs:
>>> http://lilypond.org/doc/v2.19/Documentation/extending/scheme-function-definitions
>> 
>> The next one tells the story about optional arguments:
>> &lt;URL:http://lilypond.org/doc/v2.19/Documentation/extending/scheme-function-usage&gt;
>
> Ok, thanks, I see how it works now.  
>
> So if I understand this[1] correctly...  then the way that LilyPond knows an
> optional argument is being omitted is by getting an argument of a different
> type in place of that optional argument (e.g. a string rather than a
> number).  An optional argument has to be followed by an argument of a
> different type for things to work when it is omitted.

Or be skipped using \default.

> It clearly won't work if you have two argument of the same type in a
> row and try to omit the first one as optional.  (And there's also the
> case of omitting several optional arguments in a row...)
>
> [1] "Once an optional argument predicate does not match an argument,
> LilyPond skips this and all following optional arguments, replacing them
> with their specified default, and ‘backs up’ the argument that did not match
> to the place of the next mandatory argument. Since the backed up argument
> needs to go somewhere, optional arguments are not actually considered
> optional unless followed by a mandatory argument."

The "once one optional argument is skipped" semantics was required to
get things like \key and \mark work in a sensible manner when converted
to functions from builtin syntax constructs.

-- 
David Kastrup