Re: Replace trivial pcase occurrences in the Emacs sources

[Stefan Monnier]

>> LGTM, thanks.  We could maybe give a hint about why, e.g. saying that
>> there are no guarantees about the order and number of times they are run.
> Are these the only restrictions?

I think so, yes.

> Are there even cases where side effects can still be relied on -
> e.g. in
>
> #+begin_src emacs-lisp
> (pcase EXPR
>   ((and P1 (guard (prog1 t (setq x 15))) P2) BODY1)
>   ...)
> #+end_src
>
> is it legitimate to assume that x has been set to 15 if BODY1 is
> evaluated (and P1 and P2 don't touch x)?

Yes: pcase doesn't look inside the `guard`s so in order to get to BODY1
it had to evaluate `(prog1 t (setq x 15))` at some point.

But some of the preds and guards of other branches (both before and
after the one of BODY1) may also be run before getting to BODY1.


        Stefan