From: "Drew Adams" <drew.adams@oracle.com>
To: "'Lars Magne Ingebrigtsen'" <larsi@gnus.org>,
"'Adam Sjøgren '" <asjo@koldfront.dk>
Cc: 9082@debbugs.gnu.org
Subject: bug#9082: Add hints to documentation of car and cdr for (e)lispnewcomers
Date: Thu, 14 Jul 2011 14:20:10 -0700 [thread overview]
Message-ID: <1256EB77949E454AA43F66126A20F749@us.oracle.com> (raw)
In-Reply-To: <m3fwm8y4wu.fsf@quimbies.gnus.org>
> It's really hard to explain these two functions, because they are so
> simple and have little intrinsic meaning. Which is why they
> were called why they're called, and not `first' and `rest' (which also has
> proponents).
No, that is not the reason why they were called `car' and `cdr'.
http://en.wikipedia.org/wiki/CAR_and_CDR
And no, it's not difficult to describe these two functions.
It's nearly enough to say:
1. `cons' creates a pair (called a "cons cell" or a "cons") from its two
arguments.
2. (car (cons a b)) = a
(cdr (cons a b)) = b
That's in fact the definition, and it's a pretty good explanation too.
I say "nearly" enough because it also helps to explain more about conses and
list structure (what's shared, what is not, etc.).
Lisp is not a purely functional language, so while #2 above defines these two
functions functionally, it does not describe them in terms of behavior. Mainly
because it begs the question of `cons', which is not a pure function and which
is where list structure comes into the picture.
If `cons' were simply a constructor in the functional language (or rewrite or
algebra) sense of that word (essentially an undefined function), then #2 above
would suffice.
> If a user sees
> (setq a '(foo . bar))
> (zot (cdr a))
> I don't see how looking up `cdr' and seeing "(rest)" really
> unconfuses all that much. Pointing to the manual is the only
> thing that will help here, in my opinion.
Here, I agree with you, Lars.
`rest' does not describe `cdr', especially when `rest' is thought of in terms of
purely functional languages. Again, the nub is `cons' and the behavior.
It is also true as has been pointed out that the word `rest' is a better fit for
true lists than for conses.
prev parent reply other threads:[~2011-07-14 21:20 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-14 15:03 bug#9082: Add hints to documentation of car and cdr for (e)lisp newcomers Adam Sjøgren
2011-07-14 18:31 ` Lars Magne Ingebrigtsen
2011-07-14 19:19 ` Adam Sjøgren
2011-07-14 19:47 ` Glenn Morris
2011-07-14 20:09 ` Lars Magne Ingebrigtsen
2011-07-14 20:28 ` Adam Sjøgren
2011-07-14 20:37 ` Lars Magne Ingebrigtsen
2011-07-14 21:20 ` Drew Adams [this message]
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