Re: Moving point around empty overlays with 'after-text

[Ash <ext0l@catgirl.ai>]

On Sun, Apr 9, 2023, at 7:00 PM, Platon Pronko wrote:
> On 2023-04-10 04:44, Ash wrote:
> > Yeah, I think doing this "right" might require adding a new property to overlays/strings (or giving an existing property a new value) to enable this behavior and modifying C code. Not sure how viable that is or if it's something the devs would want.
> 
> I think it's even worse than adding a property to the overlay. You need common point manipulation functions to account for possibility of inlays, i.e. (point) for position before and after inlay will be returning different values, (forward-char) will correctly advance the point from the left side to the right side of the inlay, etc.
> 
> (on second thought, making (point) return different values for positions around overlays sounds horrifying, because this will break about half of all Elisp code written)
> 
> But inlay hints seem to be a common functionality for any modern IDE nowdays, so it might make sense to support them natively, without making major-mode developers resort to horrible hacks like described before.
> 
> -- 
> Best regards,
> Platon Pronko
> PGP 2A62D77A7A2CB94E
> 

You could make it so only these special overlays (I'm going to call them inlay-type overlays or just inlays) have weird behavior with (point), but that'd still make things very complicated and I wouldn't do it. A sketch for what I would do is something like this:

When point is on a position with an inlay, the new variable point-is-after-inlay (name subject to bikeshedding) controls where point renders (before if nil, after if t). When inserting text, the inlay moves forward if point-is-after-inlay is nil, and doesn't move if it's t; this means characters appear where you'd expect.

We add a new (forward-char-respecting-inlay) function that sets point-is-after-inlay to t if point is at the start of an inlay *and* point-is-after-inlay is nil, or increments point if not (and the same for backwards). Possibly add an 'always-respect-inlay' mode that makes forward-char act like this, with the caveat that things might break in strange ways.

Each inlay has a 'bias' of 'before or 'after that indicates what point-is-before-inlay should be set to when navigating to it 'from afar' or other cases where there's no good heuristic for where to put it; in general, this should correspond to where the text is the inlay is semantically annotating.

There's probably all kinds of edge cases I haven't thought about, of course. Conversely, an even more general approach that would support multiple inlays in a row would be to have point-is-after-inlay be an *index* (and rename it 'point-inlay-index' or some such). Not sure what a concrete use case for that would be.