Re: How to use optional arguments / parameters in a define-markup-command

[Urs Liska]

5. November 2019 11:19, "Thomas Morley" <address@hidden> schrieb:

> Am Di., 5. Nov. 2019 um 09:14 Uhr schrieb Karsten Reincke <address@hidden>:
> 
>> On Mon, 2019-11-04 at 23:06 +0100, Thomas Morley wrote:
>> Am Mo., 4. Nov. 2019 um 18:00 Uhr schrieb Karsten Reincke <address@hidden>:
>> [...]
>> Let me quote another part of my reply:
>> 
>> Am Fr., 1. Nov. 2019 um 16:01 Uhr schrieb Thomas Morley
>> <address@hidden>:
>> 
>>> For variable amount of args I'd go for list? (or the like) and let the
>>> body of your code sort it out.
>> 
>> And that's basically what you do in your example-code.
>> 
>> You are totally correct. Unfortunately, I did not read your mail as 
>> thoroughly as
>> it should had been done. So, I had to find th solution by myself. But of 
>> course.
>> it stays your idea.
> 
> Well, not my idea, I just pointed to that coding-principle
> 
>>> #(define (assign keyValue assocList defaultValue)
>>> (string? list?)
>> ^^^^^^^^^^^^^^^^^^
>> As far as I can tell this line is superfluous, returning #f.
>> 
>> Yep, you are right. Due to the fact, that I later on decided also to allow 
>> other
>> default values than strings, I erased the third type test without 
>> considering that
>> then a third type is missed. So it is indeed better to erase the complete 
>> line.
> 
> You miss the point.
> There is no type-checking of this kind in guile-definitions.
> The line (string? list?) is an expression of its own, returning #f.
> If you try three elements there like in
> (define (x a b c) (string? list? string?) (list a b c))
> you'll get an error.
> 
> Type-checking of this kind is done in _LilyPond's_ functions and for
> markup-commands.
> Don't confuse them.

To be more explicit:

  (string? list?)

is an expression that calls the procedure `string?` and passes it the value 
`list?`.
What this effectively does is to check whether `list?` is a string, and since 
`list?` is a procedure itself the result of the evaluation is `#f`.

Urs
> 
> Cheers,
> Harm