From: Mark Rosenberg
Subject: Beginner Question ?
Date:
Message-ID: <11521602MR@MSU>
I'm just starting to lean lisp and have a quick question, what does CAR and CDR
stand for ? I can understand LOOP, SETQ, IF, etc... but CAR & CDR don't seem
to relate their functionality.....
Thanks,
Mark Rosenberg <·······@MSU>
Education Systems Development
Michigan State University
In article <··········@MSU> ·······@MSU.BITNET (Mark Rosenberg) writes:
>what does CAR and CDR stand for ?
They are remnants of the first Lisp implementation, for the IBM 709
(or maybe 704?), and were the opcodes used on that machine to
implement these operations. They stand for Contents of Address Register and
Contents of Decrement Register. The address register and decrement
register were the two halves of indirect pointers in memory, which
were used to implement cons cells.
Be thankful Lisp wasn't first implemented on the PDP-10, or I think
they'd be named things like HRRZ and HLRZ.
>but CAR & CDR don't seem to relate their functionality.....
OK, what would you call them? The existing names are perfect
precisely because they are meaningless! FIRST and REST are equivalent
to CAR and CDR, but only make sense when the cons cell is being
treated as the head of a list; they aren't appropriate for a tree
structure. LEFT/RIGHT or CONS-PART-1/CONS-PART-2 imply a spatial
relationship or ordering that is not inherent in the objects (Hebrew-
and Arabic-speaking people might prefer to draw their cons cell chains
from right to left, so the CAR would be on the right, but I learned to
draw them with the CAR on the left).
A cons is just an object with two cells, and it can be used to build
arbitrary structures. Since there is no forced distinction between
the two cells (except that they be separate from each other), the
names we give them are arbitrary. Perhaps we should call them
BROTHER-DARYL and OTHER-BROTHER-DARYL (comments to rec.arts.tv).
Barry Margolin
Thinking Machines Corp.
······@think.com
{uunet,harvard}!think!barmar
In article <·····@news.Think.COM> ······@think.COM (Barry Margolin) writes:
>In article <··········@MSU> ·······@MSU.BITNET (Mark Rosenberg) writes:
>>what does CAR and CDR stand for ?
>[Explanation of source of the names]
And of course, car and cdr chains are marginally pronouncable:
cadr = Cad-der
cdar = could-ar
etc.
John
·······@m2.csc.ti.com
As I heard it, CAR is Contents of Address Register, and CDR is Contents of
Decrement Register. These were hardware registers used in one of the early
implementations of Lisp.
Bill St. Clair (····@CAMBRIDGE.APPLE.COM)