bug#25890: Default value of digits-per-component?

[Clément Pit-Claudel]

Hi all,

> When I worked on the change, it seemed harmless: the function has only
> one caller outside of color.el, and I changed that single caller to
> use the optional argument.

This is a public function, with many callers outside of Emacs, it seems 
(https://github.com/search?l=Emacs+Lisp&q=color-rgb-to-hex&type=Code&utf8=%E2%9C%93);
 it will certainly break some of these uses (it did break my uses of it :/) I 
tend to follow the ChangeLogs and emacs-devel discussions, but I didn't see it 
mentioned there; maybe it would be good to?

> If the change I made is nevertheless deemed too drastic, then what are
> our alternatives?  The only one I could think of is to define a new
> function and deprecate color-name-to-rgb in favor of that new
> function, which will then display warnings when code using it is
> compiled, and eventually cause them to make changes in their code
> anyway.  Is that better?  Or are there any better ideas?

It sounds much better to me: the current solution breaks all code silently, 
while a new function doesn't break any code and me authors time to adjust.  
Additionally, with a new function, it's easy to write code that's compatible 
with all versions of Emacs (I just check whether the new function is fboundp).  
With the current change, I have to inspect the function's signature to 
determine how to call it (is there a simpler way?).

An alternative is to advertise the argument as required, default to '2', and 
issue a warning when that argument isn't explicitly specified — a bit like what 
was done with looking-back.

Thanks!
Clément.