Quine (computing)

The name "quine" was coined by Douglas Hofstadter, in his popular 1979 science book Gödel, Escher, Bach, in honor of philosopher Willard Van Orman Quine (1908–2000), who made an extensive study of indirect self-reference, and in particular for the following paradox-producing expression, known as Quine's paradox:

"Yields falsehood when preceded by its quotation" yields falsehood when preceded by its quotation.

John von Neumann theorized about self-reproducing automata in the 1940s. Later, Paul Bratley and Jean Millo's article "Computer Recreations: Self-Reproducing Automata" discussed them in 1972.{{cite journal | last1 = Bratley

| first1 = Paul

| author-link1 = Paul Bratley

| last2 = Millo

| first2 = Jean

| title = Computer Recreations: Self-Reproducing Automata

| journal = Software: Practice and Experience

| volume = 2

| issue = 4

| year = 1972

| pages = 397–400

| doi=10.1002/spe.4380020411

| s2cid = 222194376

}}

Bratley first became interested in self-reproducing programs after seeing the first known such program written in Atlas Autocode at Edinburgh in the 1960s by the University of Edinburgh lecturer and researcher Hamish Dewar.

The "download source" requirement of the GNU Affero General Public License is based on the idea of a quine.{{cite web

| url = http://www.softwarefreedom.org/technology/blog/2007/nov/21/stet-and-agplv3/

| title = stet and AGPLv3

| access-date = June 14, 2008

| last = Kuhn

| first = Bradley M.

| author-link = Bradley M. Kuhn

| date = November 21, 2007

| publisher = Software Freedom Law Center

| archive-url = https://web.archive.org/web/20080315231323/http://www.softwarefreedom.org/technology/blog/2007/nov/21/stet-and-agplv3/

| archive-date = March 15, 2008

| url-status = dead

}}

In general, the method used to create a quine in any programming language is to have, within the program, two pieces: (a) code used to do the actual printing and (b) data that represents the textual form of the code. The code functions by using the data to print the code (which makes sense since the data represents the textual form of the code), but it also uses the data, processed in a simple way, to print the textual representation of the data itself.

Here are three small examples in Python3:

1. Example A. chr(39) == "'".

a = 'a = {}{}{}; print(a.format(chr(39), a, chr(39)))'; print(a.format(chr(39), a, chr(39)))

1. Example B. chr(39) == "'".

b = 'b = %s%s%s; print(b %% (chr(39), b, chr(39)))'; print(b % (chr(39), b, chr(39)))

1. Example C. %r will quote automatically.

c = 'c = %r; print(c %% c)'; print(c % c)

The following Java code demonstrates the basic structure of a quine.

public class Quine

{

public static void main(String[] args)

{

char q = 34; // Quotation mark character

String[] l = { // Array of source code

"public class Quine",

"{",

" public static void main(String[] args)",

" {",

" char q = 34; // Quotation mark character",

" String[] l = { // Array of source code",

" ",

" };",

" for (int i = 0; i < 6; i++) // Print opening code",

" System.out.println(l[i]);",

" for (int i = 0; i < l.length; i++) // Print string array",

" System.out.println(l[6] + q + l[i] + q + ',');",

" for (int i = 7; i < l.length; i++) // Print this code",

" System.out.println(l[i]);",

" }",

"}",

};

for (int i = 0; i < 6; i++) // Print opening code

System.out.println(l[i]);

for (int i = 0; i < l.length; i++) // Print string array

System.out.println(l[6] + q + l[i] + q + ',');

for (int i = 7; i < l.length; i++) // Print this code

System.out.println(l[i]);

}

}

The source code contains a string array of itself, which is output twice, once inside quotation marks.

This code was adapted from an original post from c2.com, where the author, Jason Wilson, posted it as a minimalistic version of a Quine, without Java comments.{{cite web |url=http://wiki.c2.com/?QuineProgram |title=Quine Program |website=wiki.c2.com}}

Thanks to new [https://openjdk.java.net/jeps/378 text blocks] feature in Java 15 (or newer), a more readable and simpler version is possible:{{Cite web|url=https://gist.github.com/destan/c0db5a237e9875a56141403aaa6cb9c7|title=Simple Java quine, self replicating (Self copying) Java code, with text blocks. This code can be run with Java 15+ or Java 13+ with special flags. License is public domain, no rights reserved}}

public class Quine {

public static void main(String[] args) {

String textBlockQuotes = new String(new char[]{'"', '"', '"'});

char newLine = 10;

String source = """

public class Quine {

public static void main(String[] args) {

String textBlockQuotes = new String(new char[]{'"', '"', '"'});

char newLine = 10;

String source = %s;

System.out.print(source.formatted(textBlockQuotes + newLine + source + textBlockQuotes));

}

}

""";

System.out.print(source.formatted(textBlockQuotes + newLine + source + textBlockQuotes));

}

}

The same idea is used in the following SQL quine:

SELECT REPLACE(REPLACE('SELECT REPLACE(REPLACE("$",CHAR(34),CHAR(39)),CHAR(36),"$") AS Quine',CHAR(34),CHAR(39)),CHAR(36),'SELECT REPLACE(REPLACE("$",CHAR(34),CHAR(39)),CHAR(36),"$") AS Quine') AS Quine

Some programming languages have the ability to evaluate a string as a program. Quines can take advantage of this feature. For example, this Ruby quine:

eval s="print 'eval s=';p s"

Lua can do:

s="print(string.format('s=%c%s%c; load(s)()',34,s,34))"; load(s)()

In Python 3.8:

exec(s:='print("exec(s:=%r)"%s)')

In many functional languages, including Scheme and other Lisps, and interactive languages such as APL, numbers are self-evaluating. In TI-BASIC, if the last line of a program returns a value, the returned value is displayed on the screen. Therefore, in such languages a program consisting of only a single digit results in a 1-byte quine. Since such code does not construct itself, this is often considered cheating.

1

In some languages, particularly scripting languages but also C, an empty source file is a fixed point of the language, being a valid program that produces no output.{{efn|1=Examples include Bash, Perl, and Python}} Such an empty program, submitted as "the world's smallest self reproducing program", once won the "worst abuse of the rules" prize in the International Obfuscated C Code Contest.{{cite web |url = http://www0.us.ioccc.org/1994/smr.hint |title = IOCCC 1994 Worst Abuse of the Rules |archive-url=https://web.archive.org/web/20201112015540/http://www0.us.ioccc.org/1994/smr.hint |archive-date=12 November 2020 |url-status=dead}} The program was not actually compiled, but used cp to copy the file into another file, which could be executed to print nothing.{{cite web |title=Makefile |url=http://www0.us.ioccc.org/1994/Makefile |website=IOCCC.org |access-date=4 April 2019 |archive-date=23 April 2019 |archive-url=https://web.archive.org/web/20190423002150/http://www0.us.ioccc.org/1994/Makefile |url-status=dead }}

Quines, per definition, cannot receive any form of input, including reading a file, which means a quine is considered to be "cheating" if it looks at its own source code. The following shell script is not a quine:

1. !/bin/sh
2. Invalid quine.
3. Reading the executed file from disk is cheating.

cat $0

A shorter variant, exploiting the behaviour of shebang directives:

1. !/bin/cat

Other questionable techniques include making use of compiler messages; for example, in the GW-BASIC environment, entering "Syntax Error" will cause the interpreter to respond with "Syntax Error".

Quine code can also be outputted visually, for example it's used to visualize the neutral zone in Yars' Revenge, along with syntactic saccharin, to obfuscate the source code.

The quine concept can be extended to multiple levels of recursion, giving rise to "ouroboros programs", or quine-relays. This should not be confused with multiquines.

This Java program outputs the source for a C++ program that outputs the original Java code.

Such programs have been produced with various cycle lengths:

• Haskell → Python → Ruby{{cite web

|url = http://blog.sigfpe.com/2008/02/third-order-quine-in-three-languages.html

|title = A Third Order Quine in Three Languages

|author = Dan Piponi

|date = 5 February 2008

}}

• Python → Bash → Perl{{cite web

|url = http://www.stratigery.com/source.html#Ouroboros

|title = Ask and ye shall receive: Self-replicating program that goes through three generations, Python, Bash, Perl

|author = Bruce Ediger

|access-date = 2011-03-17

|archive-date = 2011-02-23

|archive-url = https://web.archive.org/web/20110223164033/http://www.stratigery.com/source.html#Ouroboros

|url-status = dead

}}

• C → Haskell → Python → Perl{{cite web

|url = http://hpaste.org/43501/multiquine

|title = multiquine

|author = b.m.

|date = 1 February 2011

|archive-url = https://archive.today/20130415050710/http://hpaste.org/43501/multiquine

|archive-date = 2013-04-15

|url-status = dead

}}

• Haskell → Perl → Python → Ruby → C → Java{{cite web

|url = http://blog.sigfpe.com/2011/01/quine-central.html

|title = Quine Central

|author = Dan Piponi

|date = 30 January 2011

}}

• Ruby → Java → C# → Python{{cite web

|url = http://ruslan.ibragimov.by/20-04-2013.quine-ruby-java-c-python

|title = Quine Ruby -> Java -> C# -> Python

|author = Ruslan Ibragimov

|date = 20 April 2013

|language = ru

|access-date = 20 April 2013

|archive-date = 4 March 2016

|archive-url = https://web.archive.org/web/20160304040341/http://ruslan.ibragimov.by/20-04-2013.quine-ruby-java-c-python

|url-status = dead

}}

• C → C++ → Ruby → Python → PHP → Perl{{cite web

|url = http://golf.shinh.org/reveal.rb?Quine/shinh+%28C+C%2B%2B+Ruby+Python+PHP+Perl%29_1194650418&rb

|title = Quine by shinh (C C++ Ruby Python PHP Perl)

|author = Shinichiro Hamaji

|date = 10 November 2007

}} (this one is also a polyglot)

• Ruby → Python → Perl → Lua → OCaml → Haskell → C → Java → Brainfuck → Whitespace → Unlambda{{cite web

|url = http://asiajin.com/blog/2009/09/22/uroboros-programming-with-11-programming-languages/

|title = Uroboros Programming With 11 Programming Languages

|author = Ku-ma-me

|date = 22 September 2009

|access-date = 17 March 2011

|archive-date = 29 August 2011

|archive-url = https://web.archive.org/web/20110829204605/http://asiajin.com/blog/2009/09/22/uroboros-programming-with-11-programming-languages/

|url-status = dead

}}

• Ruby → Scala → Scheme → Scilab → Shell (bash) → S-Lang → Smalltalk → Squirrel3 → Standard ML → ... → Rexx (128 (and formerly 50) programming languages){{cite web

|url = https://github.com/mame/quine-relay

|title = Quine Relay - An uroboros program with 100+ programming languages

|author = Yusuke Endoh

|website = GitHub

|date = 2 November 2021

}}

• Web application → C (web application source code consists of HTML, JavaScript, and CSS){{cite web

|url = http://michaelwehar.com/quines/c_prints_javascript.html

|title = C Prints JavaScript

|author = Michael Wehar

|date = 10 November 2019

|language = en

}}

David Madore, creator of Unlambda, describes multiquines as follows:{{cite web

|url = http://www.madore.org/~david/computers/quine.html

|title = Quines (self-replicating programs)

|author = David Madore

}}

"A multiquine is a set of r different programs (in r different languages – without this condition we could take them all equal to a single quine), each of which is able to print any of the r programs (including itself) according to the command line argument it is passed. (Cheating is not allowed: the command line arguments must not be too long – passing the full text of a program is considered cheating)."

A multiquine consisting of 2 languages (or biquine) would be a program which:

• When run, is a quine in language X.
• When supplied with a user-defined command line argument, would print a second program in language Y.
• Given the second program in language Y, when run normally, would also be a quine in language Y.
• Given the second program in language Y, and supplied with a user-defined command line argument, would produce the original program in language X.

A biquine could then be seen as a set of two programs, both of which are able to print either of the two, depending on the command line argument supplied.

Theoretically, there is no limit on the number of languages in a multiquine.

A 5-part multiquine (or pentaquine) has been produced with Python, Perl, C, NewLISP, and F#{{cite web

|url = https://github.com/rvantonder/pentaquine

|title = Pentaquine - 5 part multiquine

|author = Rijnard van Tonder

|website = GitHub

|date = 14 January 2020

}}

and there is also a 25-language multiquine.{{cite web

|url = https://github.com/coolwanglu/quine-chameleon#variants

|title = Quine Chameleon#Variants

|author = Lu Wang

|website = GitHub

|date = 21 May 2021

}}

Similar to, but unlike a multiquine, a polyglot program is a computer program or script written in a valid form of multiple programming languages or file formats by combining their syntax. A polyglot program is not required to have a self-reproducing quality, although a polyglot program can also be a quine in one or more of its possible ways to execute.

Unlike quines and multiquines, polyglot programs are not guaranteed to exist between arbitrary sets of languages as a result of Kleene's recursion theorem, because they rely on the interplay between the syntaxes, and not a provable property that one can always be embedded within another.

A radiation-hardened quine is a quine that can have any single character removed and still produces the original program with no missing character. Of necessity, such quines are much more convoluted than ordinary quines, as is seen by the following example in Ruby:{{cite web

| url = https://github.com/mame/radiation-hardened-quine

| title = Radiation-hardened Quine

| access-date = 2014-02-24

| author = Yusuke Endoh

| website = GitHub

}}

eval='eval$q=%q(puts %q(10210/#{1 1 if 1==21}}/.i rescue##/

1 1"[13,213].max_by{|s|s.size}#"##").gsub(/\d/){["=\47eval$q=%q(#$q)#\47##\47

",:eval,:instance_,"||=9"][eval$&]}

exit)#'##'

instance_eval='eval$q=%q(puts %q(10210/#{1 1 if 1==21}}/.i rescue##/

1 1"[13,213].max_by{|s|s.size}#"##").gsub(/\d/){["=\47eval$q=%q(#$q)#\47##\47

",:eval,:instance_,"||=9"][eval$&]}

exit)#'##'

/#{eval eval if eval==instance_eval}}/.i rescue##/

eval eval"[eval||=9,instance_eval||=9].max_by{|s|s.size}#"##"

Using relational programming techniques, it is possible to generate quines automatically by transforming the interpreter (or equivalently, the compiler and runtime) of a language into a relational program, and then solving for a fixed point.{{Cite book |last1=Byrd |first1=William E. |last2=Holk |first2=Eric |last3=Friedman |first3=Daniel P. |chapter=MiniKanren, live and untagged: Quine generation via relational interpreters (Programming pearl) |date=2012-09-09 |title=Proceedings of the 2012 Annual Workshop on Scheme and Functional Programming |chapter-url=http://webyrd.net/quines/quines.pdf |series=Scheme '12 |location=New York, NY, USA |publisher=Association for Computing Machinery |pages=8–29 |doi=10.1145/2661103.2661105 |isbn=978-1-4503-1895-2}}

{{Portal|Computer programming}}

{{cols}}

• Diagonal lemma
• Droste effect
• Fixed point combinator
• Self-modifying code
• Self-interpreter
• Self-replicating machine
• Self-replication
• Self-relocation
• TiddlyWiki
• Tupper's self-referential formula
• Programming languages
• Quine's paradox
• Polyglot (computing)

{{colend}}

{{Notelist|30em}}

{{Reflist|30em}}

• Douglas Hofstadter: Gödel, Escher, Bach: An Eternal Golden Braid
• Ken Thompson: "[https://www.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf Reflections on Trusting Trust]" (Communications of the ACM, 27(8):761-3)

{{Div col|colwidth=30em}}

• [http://tiddlywiki.com/#Quine:Quine%20HelloThere%20GettingStarted%20Community TiddlyWiki, a quine manifested as a wiki]
• [http://www.nyx.net/~gthompso/quine.htm The Quine Page (by Gary P. Thompson)]
• [http://michaelwehar.com/quines/ A Brief Guide to Self-Referential Programs]
• [http://c2.com/cgi/wiki?QuineProgram QuineProgram at the Portland Pattern Repository Wiki]
• [http://www.madore.org/~david/computers/quine.html David Madore's Discussion of Quines]
• [http://www.steike.com/code/useless/zip-file-quine/ Zip File Quine]
• [http://research.swtch.com/2010/03/zip-files-all-way-down.html Zip Files All The Way Down]
• [http://johannesloetzsch.de/software/quines/ An Introduction to Quines — in particular, quines using more than one language]
• [http://www.win.tue.nl/~wstomv/edu/javascript/quine.html Quine Web Page: A standards-conforming HTML+JavaScript web page that shows its own source code]
• [https://no-gravity.github.io/html-quine/ HTML Quine: An HTML page that only uses HTML and CSS to show its own source code]
• [http://www.win.tue.nl/~wstomv/edu/javascript/challenge.html Quine Challenge for Tom's JavaScript Machine], with a series of interactive hints
• [http://www-verimag.imag.fr/~monniaux/download/JavaQuine.zip A Java Quine built straight from Kleene's fixed point theorem, composition and s-n-m]
• [http://quaxio.com/qrquine/ A QR code quine]

{{Div col end}}

{{Standard test item}}

{{DEFAULTSORT:Quine (Computing)}}

Category:Source code

Category:Articles with example C code

Category:Willard Van Orman Quine

Category:Test items in computer languages

Category:Computer programming folklore

Category:Self-replication