Re: pcase defuns

[Stefan Monnier]

> As a part of a personal project, I wrote a way to define functions in an
> equivalent way to pcases.  For example:
>
> (pcase-defun mytest (a b _)
>  "Match on 'a 'b with the third argument a wildcard"
>  "a b match")
>
> (pcase-defun mytest (c ,var _)
>  "Match on 'c binding VAR, with the third argument a wildcard"
>  (format "c %s match" var) )
>
> (mytest 'a 'b 'c) -> "a b match"
> (mytest 'c 100 'c) -> "c 100 match"
>
> This is all accomplished by a few small but tricky macros and a hashtable
> that holds all the rules.

This kind of design crossed my mind a few times but I couldn't come up
with a way to give it a reasonable semantics and implementation.
Beside the issue of precedence/ordering already mentioned by Tassilo,
there's the issue of scoping and order/timing of macroexpansion.  E.g.:

    (let ((x 0))
      (pcase-defun mytest (inc-x)
        (setq x (1+ x)))
      (pcase-defun mytest (get-x)
        x))
    
    (let ((y 0))
      (pcase-defun mytest (inc-y)
        (setq y (1+ y)))
      (pcase-defun mytest (get-y)
        y))

Does this work "right" with your code?

Or:

    (eval-when-compile (require 'cl-lib))

    (pcase-defun foo (..)
      ..
      (cl-incf ..)
      ..)

is `cl-incf` properly macroexpanded during compilation of the file, or
is it delayed to when the file is loaded, at which point the `cl-incf`
macro may be undefined?


        Stefan