quine

Quine

Willard van Orman. 1908--2000, US philosopher. His works include Word and Object (1960), Philosophy of Logic (1970), The Roots of Reference (1973), and The Logic of Sequences (1990)

quine

(programming)/kwi:n/ (After the logician Willard V. Quine,via Douglas Hofstadter) A program that generates a copy of itsown source text as its complete output. Devising the shortestpossible quine in some given programming language is a commonhackish amusement.

In most interpreted languages, any constant, e.g. 42, is aquine because it "evaluates to itself". In certain Lispdialects (e.g. Emacs Lisp), the symbols "nil" and "t" are"self-quoting", i.e. they are both a symbol and also the valueof that symbol. In some dialects, the function-formingfunction symbol, "lambda" is self-quoting so that, whenapplied to some arguments, it returns itself applied to thosearguments. Here is a quine in Lisp using this idea:

((lambda (x) (list x x)) (lambda (x) (list x x)))

Compare this to the lambda expression:

(\\ x . x x) (\\ x . x x)

which reproduces itself after one step of beta reduction.This is simply the result of applying the combinator fixto the identity function. In fact any quine can beconsidered as a fixed point of the language's evaluationmechanism.

We can write this in Lisp:

((lambda (x) (funcall x x)) (lambda (x) (funcall x x)))

where "funcall" applies its first argument to the rest of itsarguments, but evaluation of this expression will neverterminate so it cannot be called a quine.

Here is a more complex version of the above Lisp quine, whichwill work in Scheme and other Lisps where "lambda" is notself-quoting:

((lambda (x)(list x (list (quote quote) x)))(quote(lambda (x)(list x (list (quote quote) x)))))

It's relatively easy to write quines in other languages suchas PostScript which readily handle programs as data; muchharder (and thus more challenging!) in languages like Cwhich do not. Here is a classic C quine for ASCIImachines:

char*f="char*f=%c%s%c;main() printf%c";main()printf

For excruciatingly exact quinishness, remove the interior linebreak. Some infamous Obfuscated C Contest entries have beenquines that reproduced in exotic ways.

Ken Thompson's back door involved an interesting variantof a quine - a compiler which reproduced part of itself whencompiling (a version of) itself.

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Synonyms for Quine

noun United States philosopher and logician who championed an empirical view of knowledge that depended on language (1908-2001)

Synonyms