Re: pcase defuns

[Andrew Hyatt]

On Thu, Mar 31, 2022 at 12:27 AM Richard Stallman <rms@gnu.org> 
wrote: 

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>   > The way I look at it, it does do this, but it's just 
>   > structured  differently. The normal defun as one arglist. 
>   > Mine has one per  matching clause, which means that it can 
>   > take a variety of  different arguments, all matching. And 
>   > the arglist and the  matching clauses are the same thing, so 
>   > the arglist can be (n), or  ('foo n), or (1 2 (3 n)), etc. 
>   > Yes, it's weird, but I think the  differentiation here is 
>   > useful, see my next point. 
>
> The argument list is not just a concept in users' minds.  The 
> command C-h f, which shows documentation for a function, 
> displays the function's argument list.  If you try C-h f cons 
> RET, you'll see what I mean. 
>
> See also `func-arity'. 
>
> So there is a practical reason for defining constructs to set up 
> the function's argument list. 
>
>   > In particular, with your  proposal, we lose the ability to 
>   > have fairly different arg  patterns, with different numbers 
>   > of args, or different  destructuring patterns. 
>
> Not necessarily.  You could specify `&rest args' for an arglist 
> that conveys no information.  But users find it useful to see in 
> a simple way what the arguments of the function should be.

Thanks for the suggestion.  I've implemented a new version along 
the lines you suggested.  So now things look like:

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

and

(defun-pattern repeatedp (&rest args)
   "Test for repeated pattern, returns nil, 'once, or 'twice, or 
'split."
   ((a a) 'once)
   ((a a a) 'twice)
   ((a b a) 'split))

With this, I think I've answered your objections.  Do you have an
opinion on what I should do with this?  I can check this in the emacs
source code, as a GNU elpa package, or just put it in an external repository.
> --
> Dr Richard Stallman (https://stallman.org)
> Chief GNUisance of the GNU Project (https://gnu.org)
> Founder, Free Software Foundation (https://fsf.org)
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