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Lexikon

quine

/kwi:n/ (After the logician Willard V. Quine, via Douglas Hofstadter) A program that generates a copy of its own source text as its complete output. Devising the shortest possible quine in some given programming language is a common hackish amusement. In most interpreted languages, any constant, e.g. 42, is a quine because it "evaluates to itself". In certain ef="module.php?name=Lexikon&file=search&eid=1&query=Lisp">Lisp dialects (e.g. ef="module.php?name=Lexikon&file=search&eid=1&query=emacs Lisp">emacs Lisp), the symbols "nil" and "t" are "self-quoting", i.e. they are both a symbol and also the value of that symbol. In some dialects, the function-forming function symbol, "lambda" is self-quoting so that, when applied to some arguments, it returns itself applied to those arguments. Here is a quine in ef="module.php?name=Lexikon&file=search&eid=1&query=Lisp">Lisp using this idea: ((lambda (x) (list x x)) (lambda (x) (list x x))) Compare this to the ef="module.php?name=Lexikon&file=search&eid=1&query=lambda expression">lambda expression: ( x . x x) ( x . x x) which reproduces itself after one step of ef="module.php?name=Lexikon&file=search&eid=1&query=beta reduction">beta reduction. This is simply the result of applying the ef="module.php?name=Lexikon&file=search&eid=1&query=combinator">combinator ef="module.php?name=Lexikon&file=search&eid=1&query=fix">fix to the ef="module.php?name=Lexikon&file=search&eid=1&query=identity function">identity function. In fact any quine can be considered as a ef="module.php?name=Lexikon&file=search&eid=1&query=fixed point">fixed point of the language' s evaluation mechanism. We can write this in ef="module.php?name=Lexikon&file=search&eid=1&query=Lisp">Lisp: ((lambda (x) (funcall x x)) (lambda (x) (funcall x x))) where "funcall" applies its first argument to the rest of its arguments, but evaluation of this expression will never terminate so it cannot be called a quine. Here is a more complex version of the above Lisp quine, which will work in Scheme and other Lisps where "lambda" is not self-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 such as ef="module.php?name=Lexikon&file=search&eid=1&query=PostScript">PostScript which readily handle programs as data much harder (and thus more challenging!) in languages like ef="module.php?name=Lexikon&file=search&eid=1&query=C">C which do not. Here is a classic ef="module.php?name=Lexikon&file=search&eid=1&query=C">C quine for ef="module.php?name=Lexikon&file=search&eid=1&query=ASCII">ASCII machines: char*f="char*f=%c%s%c main() ef="f,34,f,34,10">printf%c" main()ef="f,34,f,34,10">printf For excruciatingly exact quinishness, remove the interior line break. Some infamous ef="module.php?name=Lexikon&file=search&eid=1&query=Obfuscated C Contest">Obfuscated C Contest entries have been quines that reproduced in exotic ways. ef="module.php?name=Lexikon&file=search&eid=1&query=Ken Thompson">Ken Thompson' s ef="module.php?name=Lexikon&file=search&eid=1&query=back door">back door involved an interesting variant of a quine - a compiler which reproduced part of itself when compiling (a version of) itself. [ef="module.php?name=Lexikon&file=search&eid=1&query=Jargon File">Jargon File] (1995-04-25)

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