Re: pcase defuns

[Andrew Hyatt]

On Sun, Dec 19, 2021 at 12:23 PM Stefan Monnier 
<monnier@iro.umontreal.ca> wrote: 

> > 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?

Great points, thank you! Indeed, both of these wouldn't work - I'm 
just quoting and storing the function body for later.  I'll have 
to think about whether there is something more clever I can do 
that would fix these problems.

Does every defun-like thing obey these rules?  I'm not familiar 
with the range of them, so I'm not sure if this would be violating 
convention or not.  If so, one thing I can do is to try to 
distance this from defun perhaps with a different name such as 
`pcase-pattern', and making calls go through a function such as 
`pcase-call'.

>         Stefan