bug#13126: 24.3.50; (WISH) Document pcase in Info manual

[Stefan Monnier]

> Thanks.  `pcase' seemed a good replacement for `case'.

While it's not a plug-in replacement, it provides a superset of the
features of case, yes.

> 1. pcase-let, pcase-let*, pcase-dolist (maybe)

pcase-dolist shouldn't be documented (yet?).  Maybe pcase-let and
pcase-let* should be there, indeed.

> 2. It took some effort to understand that there is a U-PATTERN and a
>    UPATTERN.  We don't read out `*-*', do we?
>     ,----
>     |    There are two kinds of patterns involved in `pcase', called
>     | _U-patterns_ and _Q-patterns_.  The UPATTERN mentioned above are
>     | U-patterns and can take the following forms:
>     `----

What do you suggest instead?

> 3. Let's take the example of how
>     ,----
>     | (defun evaluate (exp env)
>     |   (pcase exp
>     | ,----
>     | |     (`(add ,x ,y)         (+ (evaluate x env) (evaluate y env)))
>     | `----
>     |     (`(call ,fun ,arg)    (funcall (evaluate fun) (evaluate arg env)))
>     |     (`(fn ,arg ,body)     (lambda (val)
>     |                             (evaluate body (cons (cons arg val) env))))
>     |     ((pred numberp)       exp)
>     |     ((pred symbolp)       (cdr (assq exp env)))
>     |     (_                    (error "Unknown expression %S" exp))))
>     `----
>              `(add ,x ,y)
>         `   QPATTERN

I don't know what you wanted to say here.


        Stefan