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A protest against pcase, pcase-let, pcase-let*

From: Alan Mackenzie
Subject: A protest against pcase, pcase-let, pcase-let*
Date: Tue, 31 Mar 2015 18:00:29 +0000
User-agent: Mutt/1.5.23 (2014-03-12)

Hello, Emacs.

Can we please have a moratorium on the use of pcase, etc.?  Their use is
gradually proliferating through Emacs, yet they are not documented.

OK, maybe pcase itself has a page in the elisp manual, but this page is
very difficult to understand, certainly for me.  I have attempted quite
a lot of times to make sense of it, and failed.

There are two possibilities here: (i) the page is difficult because
pcase is itself difficult.  In this case we should stop using pcase and
systematically remove it from Emacs source.  (ii) The page is difficult
because it is not well written.  In this case it should be improved.
Personally, I think (ii) is more likely to be true than (i).

pcase-let and pcase-let* are totally absent from the elisp manual.
Their doc strings say nothing more than "Like `let' but where you can
use `pcase' patterns for bindings.", without giving any clue as to what
"`pcase' patterns" are, or what the syntax and semantics of their
"use" of them for bindings look like.

Recently, jit-lock.el was changed to include a pcase-let and a
pcase-let*.  The pertinent code is, to me, completely obscure.  As these
forms encroach on an ever increasing portion of Emacs, the part of Emacs
in which I can usefully hack diminishes correspondingly.  I suspect I am
not alone here.

As an aside, I suspect that edebug will not be useful in (possibly
large) uses of these forms, given that they are implemented as macros
rather than special forms.

Could we have a moratorium on further use of these three forms until the
above issues have been resolved?

Alan Mackenzie (Nuremberg, Germany).

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