Re: Error with default value on predicate on 2.22.0

[Lukas-Fabian Moser]

Hi Paolo,

Am 12.01.21 um 17:01 schrieb Paolo Prete:
> Hello,
>
> I just experienced (on Linux) that this compiles with <= 2.20
>
> --------------------------------
>
> foo = #(define-music-function (parser location col staff mus)((color? 
> red) string? ly:music?)
> #{ c' d' #})
>
> {
> \foo "bar" {}
> }
>
> --------------------------------
>
> But it doesn't compile with 2.22.0
> What's wrong?

Last May, Valentin (in 1ea236291f96dbe5) added the ability to use 
CSS-style color names (and hex color codes, and four-number RGBA colors 
with alpha channel) everywhere where previously one had to use RGB-style 
colors. (I only found out about this _great_ addition by researching 
your problem, so I'm actually quite happy right now.)

For this, the color? predicate had to be weakened in order to cover more 
possible data types: Where previously, we had

 (define-public (color? x)
  (and (list? x)
       (= 3 (length x))
       (every number? x)
       (every (lambda (y) (<= 0 y 1)) x)))

in scm/output-lib.scm, it now reads

 (define-public (color? x)
  (or
   (string? x)
   (and (list? x)
        (let ((l (length x)))
         (or (= 3 l)
              (= 4 l)))
        (every number? x)
        (every (lambda (y) (<= 0 y 1)) x))))

To wit, while a color? used to be a list of exactly three numbers in 
[0,1], now it is _either_ a string _or_ a list of three or four numbers 
in the same interval.

So, in your function definition, "bar" is now a valid color?, hence is 
taken as "col", and consequently LilyPond tries to read {} as a string?. 
I see two solutions for your problem:

a) If you only ever need rgb colors, you could define and old-style 
color? predicate and use this in your function definition.
b) You could re-order the parameters to your function in such a way that 
the next predicate after the optional color? is not a sub-predicate of 
color?. One example would be ly:music?, so the following works (I omit 
"parser"/"location"):

\version "2.22"

foo = #(define-music-function (col mus staff) ((color? red) ly:music? 
string?)
#{ c' d' #})

{
\foo {} "bar"
}

Best
Lukas