From: Stefan Monnier <monnier@iro.umontreal.ca>
To: emacs-devel@gnu.org
Subject: Re: pcase: Semantics of atom QPATS
Date: Mon, 11 Jun 2018 10:09:37 -0400 [thread overview]
Message-ID: <jwvwov5jurg.fsf-monnier+gmane.emacs.devel@gnu.org> (raw)
In-Reply-To: 87tvqbjwyf.fsf@web.de
> isn't it strange that the pattern '.7 matches .7, but `.7 causes an
> error when used?
Yes and no: the pattern '<anything> is defined to match using (equal
x '<anything>), no matter what. In contrast, ` behaves differently
depending on the type of the backquoted object and it only supports
a limited set of types (those documented in its docstring).
So it's normal for the two to behave differently.
> IMO the analogy to the list of self-quoting atoms (strings, keywords,
> integers - I'm all for that) is misguided here - something implicitly
> quoted in a backquoted pattern (i.e. something that is not unquoted)
> should behave like quoted with `'' (i.e. transformed into an `eq' test),
> everything else feels inconsistent.
[ It's an `equal` test we want, not `eq`. ]
The issue is rather that not all types might want an `equal` test.
E.g. we could accept patterns like
`#s(hash-table (data ,key1 ,val1))
Which is why I prefer signaling an error: it lets us extend the
semantics of ` without any fear of breaking backward compatibility.
But yes: it makes sense to extend pcase's ` pattern to accept floats and
match them with `equal` since it's easy and there doesn't seem to be
many other semantics to choose anyway.
Stefan
next prev parent reply other threads:[~2018-06-11 14:09 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-10 0:40 pcase: Semantics of atom QPATS Michael Heerdegen
2018-06-11 14:09 ` Stefan Monnier [this message]
2018-06-11 14:45 ` Drew Adams
2018-06-11 15:24 ` Stefan Monnier
2018-06-11 20:53 ` Michael Heerdegen
2018-06-13 3:41 ` Michael Heerdegen
2018-06-13 12:52 ` Stefan Monnier
2018-06-16 4:03 ` Michael Heerdegen
2018-06-16 6:57 ` Eli Zaretskii
2018-06-16 7:32 ` Michael Heerdegen
2018-06-16 7:49 ` Eli Zaretskii
2018-06-17 13:35 ` Michael Heerdegen
2018-06-16 13:42 ` Noam Postavsky
2018-06-16 12:57 ` Stefan Monnier
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