Skip to main content

Quine

rascal-0.40.17

Synopsis​

A self-reproducing program.

Description​

A Quine is a computer program that takes no input and produces a copy of its own source code. A quine is also called a self-replicating or self-reproducing program.

At the Quine Page you can find hundreds of quines in many different programming languages.

Learning about quines, is about learning how to quote and escape symbols in strings.

Examples​

import IO;
import String;

void quine(){
  println(program);      ❸  
  println("\"" + escape(program, ("\"" : "\\\"", "\\" : "\\\\")) + "\";");      ❹  
}

str program =      ❶  
"import IO;
import String;

void quine(){
  println(program);
  println(\"\\\"\" + escape(program, (\"\\\"\" : \"\\\\\\\"\", \"\\\\\" : \"\\\\\\\\\")) + \"\\\";\");
}

str program =";      ❷

  • ❶ A remarkable point in the code: the string variable program has as value the text of the module Quine up to here.

  • ❷ The definition of program ends here. This string has a mesmerizing amount of escapes to which we will come back in a moment.

  • ❸ The function quine prints the string program twice, here as is and this produces the program up to icon:callout[1] above.

  • ❹ Here the value of program is printed as a string (surrounded with string quotes) in order to reproduce the string value of program followed by a semi-colon (;).

Now here is the catch: We have to be very carefull in handling special characters like quote (") and backslash (\) in strings.

Let's do a simple experiment:

rascal>import IO;
ok
rascal>str greeting = "\"Good Morning, Dr. Watson\", said Holmes";
str: "\"Good Morning, Dr. Watson\", said Holmes"
---
"Good Morning, Dr. Watson", said Holmes
---
rascal>println("\"" + greeting + "\"");
""Good Morning, Dr. Watson", said Holmes"
ok

As you see, the quotes inside the string are not escaped and the result is not a legal string. So what can we do? We escape all dangerous characters in the string before printing it using the [Rascal:escape] function. It takes a string and a map of characters to be escaped and returns a result in which all escaping has been carried out. Be aware that in the map, escaping is also needed! We want to say: Escape " and replace it by \", but since both " and \ have to be escaped themselves we have to say: Escape "\"" and replace it by "\\\"". The effect is as follows:

rascal>import String;
ok
rascal>println("\"" + escape(greeting, ("\"": "\\\"")) + "\"");
"\"Good Morning, Dr. Watson\", said Holmes"
ok

And indeed, the two quotes are now properly escaped.

This is exactly what happens at line 4 in the definition of quine:

println("\"" + escape(program, ("\"" : "\\\"", "\\" : "\\\\")) + "\";");

We escape program and replace " by \", and \ by \\. The mesmerizing amount of \ characters can be explained due to escaping " and \.

Now let's put quine to the test.

rascal>quine();
import IO;
import String;

void quine(){
  println(program);
  println("\"" + escape(program, ("\"" : "\\\"", "\\" : "\\\\")) + "\";");
}

str program =
"import IO;
import String;

void quine(){
  println(program);
  println(\"\\\"\" + escape(program, (\"\\\"\" : \"\\\\\\\"\", \"\\\\\" : \"\\\\\\\\\")) + \"\\\";\");
}

str program =";
ok

If you follow this output line-by-line you will see that it is identical to the original source code of above.

Benefits​

  • Making a Quine is a fun coding puzzle

Pitfalls​

  • Nothing useful comes from making a Quine, but some fun and relaxation.