Active characters about latex3

[josephwright]

Find use cases

[wspr]

Bringing this back to life: this is like a poor man's catcodetable scheme, right? I actually think that active characters have a degree of utility in creating light-weight markup — consider what we do with \MakeShortVerb\| and so on.

So I think from a document perspective having an interface to do this is probably a good idea. Other approaches I've seen in the distant past around this revolved around using a stack to push/pop active meanings, but I don't see as an advantage there.

One wrinkle is that if we provide a "special" interface for setting active character meanings we should disable or at least check for an attempt to overwrite the fundamental definitions using more primitive commands. Not sure if that should be a check related functionality or something built into \cs_set or the xparse functions.

[josephwright]

@wspr First step is to have a fixed set of actives of course: that's often part of the issue. I do see your point about | for short verbatim but that's a rather specialised use (in the sum total of LaTeX documents, actually printing code is relatively rare), but also wonder if there is a need to turn it on-and-off ... I guess tables?

[josephwright]

We've handled active chars now using \char_generate:nn and \cs_(g)set_active:eq:NN