Re: Rationalising c[ad]\{2,5\}r.

[Alan Mackenzie]

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).