Re: Installing cond* in core

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan.

On Fri, Feb 02, 2024 at 08:17:25 -0500, Stefan Monnier wrote:
> >   > I think the question is rather: why should we have two equivalent and
> >   > almost identical matching syntaxes and implementations?
> > That is the easiest way to make cond* function.

> Only if you consider "easiest" at each small incremental step.
> The overall cost of what you've done is already much higher :-)

> > I do not understand the pcase code for implementing pattern matching,

> You don't need to understand it.

I've needed to understand it, and I also found it difficult to
understand.

I've also found difficult the macro expansions which pcase produces.
cond* might be easier here, though it's too soon to tell.

> > A priori, I suspect it won't fit.  cond*-subpat, which does this job
> > in cond*, passes data up and down the recursion.  I can't tell whether
> > pcase does something comparable.  Also, cond*-subpat does not generate
> > the overall structure of a clause -- its caller, cond*-match, does
> > that.

> Throw away cond*-subpat.  Just rewrite

>     (cond*
>      ((match* PAT FORM) THEN)
>      . ELSE)

> into something like

>     (pcase FORM
>       (PAT THEN)
>       (_ (cond* . ELSE)))

> And presumably for

>     (cond*
>      ((match* PAT FORM))
>      . REST)

> you'd rewrite it to

>     (pcase-let ((PATH FORM))
>       (cond* . REST))

> No need to look inside or understand `pcase.el` at all!

But that would rule out the possibility of simplifying pcase by
rewriting it in terms of cond*.  ;-)

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).