programming /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
Lispdialects (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)))
( x . x x) ( x . x x)
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
Cwhich do not. Here is a classic
C quine for
ASCIImachines:
For excruciatingly exact quinishness, remove the interior line
quines that reproduced in exotic ways.
of a quine - a compiler which reproduced part of itself when
compiling (a version of) itself.
(1995-04-25)