/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 Lisp
dialects (e.g. 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 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 fix
to the identity function
. In fact any quine can be considered as a fixed point
of the language's evaluation mechanism.
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 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 PostScript
which readily handle programs as data; much harder (and thus more challenging!) in languages like C
which do not. Here is a classic C
quine for ASCII
char*f="char*f=%c%s%c;main() printf(f,34,f,34,10);%c"; main()printf(f,34,f,34,10);
For excruciatingly exact quinishness, remove the interior line break. Some infamous Obfuscated C Contest
entries have been quines that reproduced in exotic ways. Ken Thompson
's back door
involved an interesting variant of a quine - a compiler which reproduced part of itself when compiling (a version of) itself.