Re: pcase defuns

[Andrew Hyatt]

Based on the results of the ensuing discussion, I decided to 
refine my macro to just be one coherent defun.  I experimented 
with a few different ways to do this, but settled on something 
that seemed relatively clear and minimal, with pattern matchers 
that seemed like a better fit for arglists than what exists in 
pcase already (but it's all pcase under the hood).

To give a flavor of what it looks like in practice, here's a few 
pattern defuns from the test:

(defun-pattern fibonacci
   "Compute the fibonacci sequence."
    ((0) 0)
    ((1) 1)
    ((n)
      (+ (fibonacci (- n 1))
           (fibonacci (- n 2)))))

(defun-pattern crit
 "D&D 5e critical hit computation, taking in the results of a d20
and a list of attributes currently in effect.  Returns whether the
results are a crit or not."
  ((20 _) t)
  ((19 (pred (member 'improved-crit))) t))

I'm attaching the package, which works, although there's some 
non-ideal parts of the implementation.  If people think it is 
valuable, I can add this to GNU ELPA.

defun-pattern.el
Description: application/emacs-lisp

defun-pattern-test.el
Description: application/emacs-lisp

On Sat, Dec 18, 2021 at 11:53 PM Andrew Hyatt <ahyatt@gmail.com> 
wrote: 

> Hi all,
>
> 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.
>
> I find this method of programming quite useful for certain kinds 
> of problems, and pcase is close to what I want, but using pcase 
> forced me to encapsulate all of my pattern matching logic in one 
> function. Being able to have it spread out in pattern matching 
> functions seems much nicer to me.
>
> If this is of interest, I can either add this to the pcase module 
> itself, create a package in gnu elpa, or (if there's some reason 
> that it shouldn't be done) just keep it in a personal repository.
>
> I'll wait for some guidance before creating a final version, with 
> tests, wherever it should go.