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Re: The poor state of documentation of pcase like things.

From: Michael Heerdegen
Subject: Re: The poor state of documentation of pcase like things.
Date: Sun, 27 Dec 2015 00:17:04 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)

Aaron Ecay <address@hidden> writes:

> Atoms (strings, numbers, and keywords) are simultaneously patterns and
> qpatterns.  That is, one writes `(1 2) and not `(,1 ,2) to match a list
> containing 1 and 2 (though actually either form works).  Why could _ not
> also be shared between the pattern and qpattern classes?

If you consider that ` quotes any element in its argument list unless it
is explicitly unquoted (that's how it actually works), an ATOM becomes
'ATOM, which is equivalent to just ATOM as a pattern.

That one can avoid the comma before atoms inside ` reflects that
equivalence on a different level.

There are more such analogies (for example `something and 'something are
equivalent if something doesn't include a comma) and it's good that they
are there, because it makes the grammar easier to memorize, aka "build a
mental model".  Maybe you can call this "aesthetics" in this regard.

Making _ a qpattern would break the logic in the design: '_ matches the
symbol _, not anything as the _ pattern does  (disjointness of
qpatterns and pcase patterns is a different thing, and I think nobody
cares about that).

Making _ a qpattern wouldn't break pcase, but it would make the thing
less consistent, because there would be a fracture somewhere in the
logic, e.g. if _ is a qpattern matching anything, `_ and '_ would have
different semantics as pcase patterns (the first would be equivalent to
_), or such things.

I would agree to do this anyway if there was a good reason.  Saving a
comma or fitting the expectations of users that haven't yet completely
understood the semantics (I don't mean Phillip by that, I mean users who
want to learn the thing) is not a sufficing reason for obfuscating the
logic behind the design, I think.

Another thing is that I think, and that's what we want to emphasize in
the docs, that qpatterns are conceptually a different thing than pcase
patterns.  Actually, the term "qpattern" has not much meaning at all, it
is only used as a helper to describe the grammar of ` (because it is
recursive, but actually, the grammar is very simple).

A qpattern is either an expression that is compared with equal, or a
placeholder for a real pattern via unquoting.  That's all.  If the
description of ` in pcase would be less formal and e.g. more like the
doc of `backquote', we probably would not even have a name for that.

Anyway, I think it's conceptually a bad thing to merge those two quite
different things.

Just my personal opinion of course.  I've worked with pcase quite a lot
now.  It took some time until I had internalized the concept (which is
why I think it's important to keep it simple).  If others who have used
the tool quite regularly do think it would be a good idea to make _ a
qpattern, let's do it.  But I think we should not hurry modifying a
thing in a way which at the first look seems like a good idea, and we
may regret later.

Sorry for the prose, this was more than you had asked for.



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