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A quine, named after the philosopher W. V. O. Quine, is a program that prints its own source code. One of the typical challenges of any new esoteric programming language is to write a quine other than the null program in it.

Often a program that just performs access to its own source code (reading it either from memory or from a disk file) and prints it out is considered "cheating".

The language Muriel is based on the idea of quines.

How to write quines

Quines can be developed in any Turing-complete language which has the ability to freely create output in its own source alphabet (though the existence of quines does not imply Turing-completeness). Despite all the mystery surrounding them, they are actually quite simple to develop. Here is an algorithm for doing so:

First we construct the function print which outputs its input. In most languages this is trivial. For example, in Javascript we have


Now consider print(print), the program which passes the source code of print to print. For example

 input = "console.log(input);";

It is quite easy to construct print(print), so we'll go ahead and write a program that given the input of the source code of program t outputs the source code of print(t). We'll call that program quinify. A quick note: you'll sometimes run into issues with escape sequences when writing quines. This is not a theoretical problem, only a language specific one. In this case we're using String.fromCharCode(34) to represent double quotes to avoid the issue. Anyway, here's quinify in Javascript:

 console.log('input = ' + String.fromCharCode(34) + input + String.fromCharCode(34) + ';' + String.fromCharCode(10) + input);

The final step is easy! Pass the source code of quinify to quinify. We have a quine.

 input = "console.log('input = ' + String.fromCharCode(34) + input + String.fromCharCode(34) + ';' + String.fromCharCode(10) + input);";
 console.log('input = ' + String.fromCharCode(34) + input + String.fromCharCode(34) + ';' + String.fromCharCode(10) + input);

Here it is minimized for those inclined (151 bytes)


Clearly the only challenge here is in writing quinify, which in many esolangs is non-trivial. Get to work!

See also

External resources