Re: Questions about text properties

[Eli Zaretskii <eliz@gnu.org>]

> From: "Garreau\, Alexandre" <galex-713@galex-713.eu>
> Date: Sun, 21 Oct 2018 05:21:40 +0200
> 
> – how about adding stuff to a *value* of property, be it a list, or
>   anything that could inherit, such as an alist, a keymap, etc.? it
>   seems there are no functions for that;

Not sure I understand why you need something special: modifying a Lisp
value, whether to add something or otherwise, is rather trivial, no?

> – why are properties rear-sticky by default rather than the opposite?

Because the other alternative is rarely needed.  E.g., if you modify a
string that has a face, you want the appended characters to inherit
the same face.  By contrast, inserting before the string rarely needs
such inheritance.

> – why is there only a mouse-face property, rather than a hover, click,
>   pressed, active, focus, etc.

The usual reason: low demand and/or low motivation to develop features
that are in low demand.  Another possible reason is that at least some
of those (click and press -- what's the difference, btw?) you can have
already, by using the 'keymap' property, and perhaps a few others.

> – why can it only affects face rather than other properties so that to
>   extend its behavior?

By "it" you mean, mouse-face?  Because it's a face.

> – sometimes most text properties can take a hook (that will be called
>   lazily to compute a valid value), sometimes not (like for a display
>   prop space);

Not sure what this is about.  In general, only functions have hooks,
not properties.  How about a couple of examples?

> — is the recenter thing about overlays related to the fact buffers are
>   implemented with a buffer gap, or is it just something similar to
>   fasten stuff?

It makes overlay access faster.