Re: master ea93326: Add `union' and `intersection' to rx (bug#37849)

[Stefan Monnier]

> An interesting suggestion, but exactly how would that model work? Consider:
>
>  (rx-define ident-chars (any "a-z0-9"))
>  (rx-define op-chars (any "+*/" ?-)
>
> To form the union of the two, 'any' needs to be allowed inside 'union':
>
>  (any (union ident-chars op-chars))

Indeed to avoid this, we'd have to do something like

    (rx-define-charset ident-chars "a-z0-9")
    (rx-define-charset op-chars (union "+*/" ?-))

and then

     ... (any (union ident-chars op-chars)) ...

Is it worth the trouble?

> Speaking of 'or', we still haven't figured out what to do about regexp-opt
> in rx. Currently, the answer is something like
> (eval (regexp-opt STRINGS)), which isn't ideal.

We could try and automatically determine inside `or` when regexp-opt can
be used.  That doesn't sound like much fun, tho.


        Stefan