From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Nathan Trapuzzano <nbtrap@nbtrap.com>
Cc: emacs-devel@gnu.org
Subject: Re: Double unquote/unquote-splicing
Date: Mon, 04 Nov 2013 13:33:15 -0500 [thread overview]
Message-ID: <jwvfvrcja45.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <874n7sf3pi.fsf@nbtrap.com> (Nathan Trapuzzano's message of "Mon, 04 Nov 2013 12:59:21 -0500")
>> 2- Since we don't allow (\` (a (\, 1 2 3))), it's weird to allow
>> ``(a ,,@x) since one possible expansion for it when x=(1 2 3) is
>> (\` (a (\, 1 2 3))).
>> Another way to say it is that we should allow (\, 1 2 3) and (\,@
>> 1 2 3), but that can't be used with the ,e and ,@e syntax, so it'll
>> stay as a second-rate citizen.
> This is incorrect. ``(a ,,@x) where x=(1 2 3) would evaluate to
> (\` (a (\, 1) (\, 2) (\, 3))). The first comma in ,,@ has the effect of
> being applied member-wise to each element spliced out of ,@. Cf. CLHS
> `Backquote' (http://www.lispworks.com/documentation/HyperSpec/Body/02_df.htm).
This is even worse since it means that the expansion of ,@foo depends on
the context.
> I believe so. (FWIW, I don't think it's that easy to tell, though I'm
> not great at this.
Back to problem nb 1, eh?
> "The backquote syntax was particularly powerful when nested. This
> occurred primarily within macro-defining macros; because such were coded
> primarily by wizards, the ability to write and interpret nested
> backquote expressions was soon surrounded by a certain mystique. Alan
> Bawden of MIT acquired a particular reputation as backquote-meister in
> the early days of the Lisp Machine." - "The Evolution of Lisp", Gabriel,
> Steele.
That sounds about right: it's only for wizards.
Nested backquotes were largely broken in Elisp and it took many years
for someone to notice. I do use them occasionally, but only in fairly
simple ways. The resulting code is largely impenetrable, so I don't
want to encourage it.
Stefan
next prev parent reply other threads:[~2013-11-04 18:33 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-04 14:03 Double unquote/unquote-splicing Nathan Trapuzzano
2013-11-04 17:27 ` Stefan Monnier
2013-11-04 17:59 ` Nathan Trapuzzano
2013-11-04 18:11 ` Nathan Trapuzzano
2013-11-04 18:33 ` Stefan Monnier [this message]
2013-11-04 19:01 ` Drew Adams
2013-11-04 19:09 ` Nathan Trapuzzano
2013-11-04 20:41 ` Stefan Monnier
2013-11-05 15:03 ` Nathan Trapuzzano
2013-11-05 19:14 ` Stefan Monnier
2013-11-05 4:01 ` Stephen J. Turnbull
2013-11-05 15:22 ` Nathan Trapuzzano
2013-11-05 21:48 ` Stephen J. Turnbull
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