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Can anyone well versed in lisp explain this joke to me? I've done some reading on functional programming languages and know that CAR/CDR mean Contents of Address/Decrement Register but I still don't really understand the humour.

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start "scheming around" a bit and then you'll understand. hahaha. it's better than a bumper sticker that says "this programmer stops at all garbage collections." –  gonzobrains Sep 29 '11 at 17:24

4 Answers 4

up vote 55 down vote accepted

In Lisp, a linked list element is called a CONS. It is a data structure with two elements, called the CAR and the CDR for historical reasons. (Some Common Lisp programmers prefer to refer to them using the FIRST and REST functions, while others like CAR and CDR because they fit well with the precomposed versions such as (CADR x) ≡ (CAR (CDR x)).

The joke is a parody of the bumper stickers you sometimes see on beat-up old cars saying "My other car is a Porsche/BMW/etc."

My response to this joke has always been "My other CAR is a CADR. CDR isn't a CAR at all."

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+1 for having a usual response. :) –  Kobi Dec 8 '09 at 5:49
very nice, but not true. Not after (rplacd a (car a)) it manifestly won't. :) Common LISP is not Haskell. But thanks for the explanation. +1. –  Will Ness May 3 '12 at 16:42
@WillNess Common Lisp is not Common LISP either. –  Miles Rout Dec 1 '14 at 21:30
I didn't want to explain my joke, but... the point is that the CDR operation isn't a CAR operation; that's a separate issue from whether or not the values are equivalent via RPLACD or whatever. –  Peter S. Housel Dec 4 '14 at 21:30

Yes, definitely a geek joke.

The names come from the IBM 704, but that's not the joke.

The joke is (bad) pun on "my other car is a ___." But the in-joke is about recursion.

When you loop/manipulate/select/invoke/more in lisp you use a combination of car (the first element in the list) and cdr (the rest of the list) to juggle functions.

So you've got a car, but your other car is your cdr because you can always get a car from a cdr since the cdr is always (in recursion) more elements. Get it? Laugh yet?

You'll probably have to learn lisp to actually chuckle a bit, or not. Of course, by then, you'll probably find yourself chuckling randomly for no apparent reason because:

Lisp makes you loopy.

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And with your last breath, another game began. –  zxq9 May 20 '14 at 4:30

//Coming from Scheme
Scheme has very few data structures, one of them is a tuple: '(first . second). In this case, car is the first element, and cdr is the second. This construct can be extended to create lists, trees, and other structures.
The joke isn't very funny.

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+1, the joke isnt very funny :P –  Russell Dec 8 '09 at 5:38
Wouldn't the tuple be '(first . second)? –  Ken Dec 8 '09 at 5:42
@Ken - again, I don't know lisp, but scheme doesn't have such a complex syntax. Even lists are made of pairs. –  Kobi Dec 8 '09 at 5:46
Indeed, it would be more accurate to say that the tuple is (first . second). The list '(first second) is made of two tuples, like this: (cons first (cons second null)) –  mquander Dec 8 '09 at 6:02
Kobi: I know Lisp, and I'm not sure what you mean by "complex syntax". Dotted syntax is how you write pairs in Lisp, including Scheme: gnu.org/software/mit-scheme/documentation/mit-scheme-ref/… . The cdr of (first second) is (second), not second. –  Ken Dec 8 '09 at 6:03

Maybe it's just an 'in-joke', only understandable by those 'in the know'.

Some would find it amusing that they understand something most readers don't.

On the other hand, if it's only seen by people who would understand it, it becomes a celebration of shared knowledge. We all like to do that, don't we?

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Not all nerd humor is reveling in specialized knowledge. –  Richard Simões Dec 8 '09 at 5:43
Do you have an example of nerd humor that doesn't revel in specialized (or shared?) knowledge? –  Jürgen A. Erhard Jan 7 '10 at 20:43
I think this question is really for BipedalShark, but it occurs to me that the Jargon File might provide a few examples, for example the nerd tendency to add '-itude' to words, and how about expressions such as 'hilarity ensues' ? –  pavium Jan 7 '10 at 23:23
I would buy this bumper sticker if ThinkGeek sold it. –  gonzobrains Sep 29 '11 at 17:25

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