Re: pcase defuns

[Andrew Hyatt]

On Sun, Dec 19, 2021 at 09:34 AM Tassilo Horn <tsdh@gnu.org> 
wrote: 

> Andrew Hyatt <ahyatt@gmail.com> writes: 
>
> Hi Andrew, 
>
> > 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" 
>
> So that's basically similar to cl-defgeneric / cl-defmethod just 
> with pcase pattern matching, right? 
>
> In that case, I think I like the cl approach better, i.e., 
> having a separate definition for the signature + several 
> implementations for variants. 
>
> But since the order of patterns is very significant, I'm not 
> sure if it's a good idea to specify them separately.  E.g., what 
> if there are pcase-defun's for mytest in several files?  Then 
> you might get different behavior depending on the order how 
> files are loaded.  And even in the "all patterns in mytest.el" 
> case: what if I re-evaluate some pcase-defuns after the file has 
> been loaded?

I have a solution for this: a crude one: I just sort all the rules 
according to their flattened length. So the most complicated ones 
should get tried first, which I think is something that can 
communicated to the user. There's other ways the system could 
behave, such as offering to the user to define a priority for the 
rule. But definitely it's easy to make sure new rules can be added 
without crazy things happening.

You've hit upon the issue with these types of systems. Powerful, 
easy to use, but dispatch rules are always vague.  The same thing 
can happen with cl-defmethod, IIRC, where you can have two rules 
that both seem like they should take precedence.  I think offering 
this functionality gives the user that choice - if they want very 
specific ordering, they can just use pcase in a function, either 
in a solution as you propose, or just use pcase itself, which is 
pretty straighforward.

> So probably I'd favor a pcase-defun which specifies all patterns and
> bodies in a single definition, e.g., like:
>
> (pase-defun mytest
>   "Docstring"
>  ((a b _)
>   "a b match")
>  ((c ,var _)
>   (format "c %s match" var)))
>
> Bye,
> Tassilo