CAR and CDR

Lisp was originally implemented on the IBM 704 computer, in the late 1950s.

The popular explanation that CAR and CDR stand for "Contents of the Address Register" and "Contents of the Decrement Register"See, for example, {{Citation |last=Mitchell |first=John C. |title=Concepts in Programming Languages |publisher=Cambridge University Press |year=2003 |isbn=9781139433488 |url=https://books.google.com/books?id=7Uh8XGfJbEIC&pg=PA29 |pages=28–29}}, Section 3.4, Innovations in the Design of Lisp. The reference identifies the IBM 704 and correctly explains the address and decrement part of a cons cell, but then it omits the "part of" in McCarthy's explanation. does not quite match the IBM 704 architecture; the IBM 704 does not have a programmer-accessible address register and the three address modification registers are called "index registers" by IBM.

The 704 and its successors have a 36-bit word length and a 15-bit address space. These computers had two instruction formats, one of which, the Type A, had a short, 3-bit, operation code prefix and two 15-bit fields separated by a 3-bit tag. The first 15-bit field was the operand address and the second held a decrement or count. The tag specified one of three index registers. Indexing was a subtractive process on the 704, hence the value to be loaded into an index register was called a "decrement".704 - electronic data-processing machine [http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/704/24-6661-2_704_Manual_1955.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/ibm/704/24-6661-2_704_Manual_1955.pdf]{{rp|p. 8}} The 704 hardware had special instructions for accessing the address and decrement fields in a word.{{rp|p. 26}} As a result it was efficient to use those two fields to store within a single word the two pointers needed for a list.{{cite web

| last = McCarthy | first = John | author-link = John McCarthy (computer scientist)

| date = 1979-02-12

| title = History of Lisp

| url = http://www-formal.stanford.edu/jmc/history/lisp/lisp.html

}}{{rp|Intro.}}

Thus, "CAR" is "Contents of the Address part of the Register". The term "register" in this context refers to "memory location".{{harvtxt|McCarthy|1960|pp=26–27}} discusses registers on the free list and in garbage collection.{{Citation |last1=McCarthy |first1=John |last2=Abrahams |first2=Paul W. |last3=Edwards |first3=Daniel J. |last4=Hart |first4=Timothy P. |last5=Levin |first5=Michael I. |title=LISP 1.5 Programmer's Manual |publisher=MIT Press |location=Cambridge, Massachusetts |year=1985 |edition=second |isbn=978-0-262-13011-0 |url=https://archive.org/details/lisp15programmer00john }}, page 36, describes cons cells as words with 15-bit "address" and "decrement" fields.

Precursors[http://dl.acm.org/citation.cfm?id=321022 A Fortran-Compiled List-Processing Language][http://www.informatimago.com/articles/flpl/flpl.html A Fortran-Compiled List-Processing Language; HTML transcription] to Lisp included functions:

• car ("contents of the address part of register number"),
• cdr ("contents of the decrement part of register number"),
• cpr ("contents of the prefix part of register number"), and
• ctr ("contents of the tag part of register number"),

each of which took a machine address as an argument, loaded the corresponding word from memory, and extracted the appropriate bits.

The 704 assembler macro for car was:

LXD JLOC 4 # C( Decrement of JLOC ) → C( C ) # Loads the Decrement of location JLOC into Index Register C

CLA 0,4 # C( 0 - C( C ) ) → C( AC ) # The AC register receives the start address of the list

PAX 0,4 # C( Address of AC ) → C( C ) # Loads the Address of AC into Index Register C

PXD 0,4 # C( C ) → C( Decrement of AC ) # Clears AC and loads Index Register C into the Decrement of AC

The 704 assembler macro for cdr was:Portions from NILS' LISP PAGES- [https://archive.today/20041021220505/http://t3x.dyndns.org/LISP/QA/carcdr.html http://t3x.dyndns.org/LISP/QA/carcdr.html]MIT AI Lab Memo 6 [https://web.archive.org/web/20170706114352/ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf https://web.archive.org/web/20170706114352/ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf]CODING for the MIT-IBM 704 COMPUTER [http://bitsavers.informatik.uni-stuttgart.de/pdf/mit/computer_center/Coding_for_the_MIT-IBM_704_Computer_Oct57.pdf http://bitsavers.informatik.uni-stuttgart.de/pdf/mit/computer_center/Coding_for_the_MIT-IBM_704_Computer_Oct57.pdf]

LXD JLOC 4 # C( Decrement of JLOC ) → C( C ) # Loads the Decrement of location JLOC into Index Register C

CLA 0,4 # C( 0 - C( C ) ) → C( AC ) # The AC register receives the start address of the list

PDX 0,4 # C( Decrement of AC ) → C( C ) # Loads the Decrement of AC into Index Register C

PXD 0,4 # C( C ) → C( Decrement of AC ) # Clears AC and loads Index Register C into the Decrement of AC

A machine word could be reassembled by cons, which took four arguments (a,d,p,t).

The prefix and tag parts were dropped in the early stages of Lisp's design, leaving CAR, CDR, and a two-argument CONS.

Compositions of car and cdr can be given short and more or less pronounceable names of the same form. In Lisp, (cadr '(1 2 3)) is the equivalent of (car (cdr '(1 2 3))); its value is 2. Similarly, (caar '((1 2) (3 4))) (pronounced {{IPAc-en|ˈ|k|eɪ|ɑr}}) is the same as (car (car '((1 2) (3 4)))); its value is 1. Most Lisps, for example Common Lisp and Scheme, systematically define all variations of two to four compositions of car and cdr.

Many languages (particularly functional languages and languages influenced by the functional paradigm) use a singly linked list as a basic data structure, and provide primitives or functions similar to car and cdr. These are named variously first and rest, head and tail, etc. In Lisp, however, the cons cell is not used only to build linked lists but also to build pair and nested pair structures, i.e. the cdr of a cons cell need not be a list. In this case, most other languages provide different primitives as they typically distinguish pair structures from list structures either typefully or semantically. Particularly in typed languages, lists, pairs, and trees will all have different accessor functions with different type signatures: in Haskell, for example, car and cdr become fst and snd when dealing with a pair type. Exact analogues of car and cdr are thus rare in other languages. Clojure uses first instead of car and next or rest instead of cdr. Logo, on the other hand, uses first instead of car and butfirst instead of cdr.

{{reflist}}

;Notes

• {{cite web |url=ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf |archive-url=https://web.archive.org/web/20170706114352/ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-006.pdf |url-status=dead |archive-date=2017-07-06 |title=Writing and Debugging Programs |last=Russell |first=Steve |author-link=Steve Russell (computer scientist) |department=RLE and MIT Computation Center |website=CSAIL Publications and Digital Archive |series=AI Memo, no. 6 |publisher=MIT Artificial Intelligence Laboratory |location=Cambridge, Massachusetts |type=Memo |oclc=35415961 |access-date=July 20, 2017}}
• {{cite web

| last = Faase | first = Frans

| date = 2006-01-10

| title = The origin of CAR and CDR in LISP

| url = http://www.iwriteiam.nl/HaCAR_CDR.html

}}

• {{cite book

| last = Graham

| first = Paul

| author-link = Paul Graham (computer programmer)

| title = ANSI Common Lisp

| publisher = Prentice Hall

| year = 1996

| isbn = 978-0-13-370875-2

| url = https://archive.org/details/ansicommonlisp00grah

}}

• {{cite book|last=Barski|first=Conrad|title=Land of Lisp : learn to program in Lisp, one game at a time! |year=2010|publisher=No Starch Press, Inc.|location=San Francisco, CA|isbn=978-1-59327-281-4}}
• {{cite journal |last=McCarthy |first=John |author-link=John McCarthy (computer scientist)|title=Recursive functions of symbolic expressions and their computation by machine, Part I.|journal=Communications of the ACM|volume=3|issue=4|year=1960|pages=184–195|url=http://jmc.stanford.edu/articles/recursive/recursive.pdf|publisher=ACM New York, NY, USA|doi=10.1145/367177.367199|s2cid=1489409}}

Category:Lisp (programming language)