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Re: Rationalising c[ad]\{2,5\}r.
From: |
Alan Mackenzie |
Subject: |
Re: Rationalising c[ad]\{2,5\}r. |
Date: |
Fri, 13 Mar 2015 16:41:24 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
Hello, Stefan.
On Thu, Mar 12, 2015 at 09:53:13AM -0400, Stefan Monnier wrote:
> > I propose the following solution: all these defuns should be in subr.el,
> Emacs lived with just car/cdr for many many years. Then someone saw
> that (car (cdr ...)) was fairly common in Elisp and decided that it was
> worth moving cl.el's cadr/caar/cadr/cddr to subr.el (tho only those,
> keeping the longer ones in cl.el).
Yes. That causes problems (even if not serious ones) because the longer
ones are loaded only with cl.
> While there are a few uses of cXXr with more than 2 Xs, these aren't
> very common, ....
They're not that rare. I count 304 currently in Emacs, including
eudc-cdaar (twice), in 78 files. That's a lot of files that need to
require cl.
> .... and I personally find them to be not terribly readable (basically,
> car/cdr feel a bit like assembly-level programming to me, since they
> access structure elements without giving them a name).
They may not be very readable, but they're more readable as "(cadar ..)"
than as "(car (cdr (car ..)))".
> Additionally, these are usually somewhat inefficient (because some of
> the inner car/cdr could/should be shared between different calls, but
> our byte-compiler doesn't know how to do
> common-subexpression-elimination, so it's better to spell them out as
> something like (cadr (cadr x)) and then to manually move the inner cadr
> to a let-binding to share it between various cXXr calls).
I think the difference in execution time would tend towards zero. Each
`car' or `cdr' is but a single compiled instruction, and let-binding
variables, then accessing them, must be quite slow by comparison.
> So while I'm not dead-set against adding many more cXXXr to subr.el, I'm
> not in favor of it, since I think it encourages a poorly-readable and
> inefficient programming style.
OK. How about us sticking with the current maximum of four?
> This said, I am in favor of moving cl--compiler-macro-cXXr to subr.el and
> making use of it (in place of inlining) for cadr/caar/cddr/cdar.
> [ It's my fault if it's not done that way yet, but that was just
> a mistake on my part. ]
I've incorporated that into the current version of my change.
> > Just one thing, though, since `number-sequence' is also defined in
> > subr.el, I'd have to wrap it in `eval-and-compile' to be able to use it
> > in my macros.
> I'm not sure it's necessary, because subr.el is preloaded (and preloaded
> early, i.e. before eager macro-expansion is enabled).
I'll try making my changes to subr.el then bootstrapping.
> Stefan
--
Alan Mackenzie (Nuremberg, Germany).
- Re: Rationalising c[ad]\{2,5\}r., (continued)
- Re: Rationalising c[ad]\{2,5\}r., Thien-Thi Nguyen, 2015/03/11
- Re: Rationalising c[ad]\{2,5\}r., Alan Mackenzie, 2015/03/11
- Re: Rationalising c[ad]\{2,5\}r., Artur Malabarba, 2015/03/11
- Re: Rationalising c[ad]\{2,5\}r., Alan Mackenzie, 2015/03/12
- Re: Rationalising c[ad]\{2,5\}r., Artur Malabarba, 2015/03/12
- Re: Rationalising c[ad]\{2,5\}r., Stefan Monnier, 2015/03/12
- Re: Rationalising c[ad]\{2,5\}r.,
Alan Mackenzie <=
- RE: Rationalising c[ad]\{2,5\}r., Drew Adams, 2015/03/13
- Re: Rationalising c[ad]\{2,5\}r., Stefan Monnier, 2015/03/13
- Re: Rationalising c[ad]\{2,5\}r., Alan Mackenzie, 2015/03/13
- Re: Rationalising c[ad]\{2,5\}r., Stefan Monnier, 2015/03/13