Question

Many results in computability theory (such as Kleene’s second recursion theorem) ensure that it is possible to construct programs that can operate over their own source code. For example, in Michael Sipser’s “Introduction to the Theory of Computation,” he proves a special case of the Recursion Theorem, which states that any program representing a function that accepts two strings and produces a string can be converted into an equivalent program where the second argument is equal to the program’s own source code. Moreover, this process can be done automatically.

The construction that one uses to produce programs with access to their own source code is well-known (most theory of computation books contain it) and is often used to generate quines. My question is whether someone has written a general-purpose tool that accepts as input a program in some language (perhaps C, for example) that contains some placeholder for the source of the program, then processes the program to produce a new program with access to its own source code. This would make it possible, for example, to generate quines automatically, or to write programs that can introspect on their syntax trees (possibly enabling reflection in languages that don’t already support it). If not, I was planning on writing my own version of such a tool, but I don’t want to reinvent the wheel if this has already been done.

EDIT: Based on @Henning Makholm’s suggestion, I decided to just sit down and implement such a program. The resulting program (which I’ve dubbed “kleene“) accepts as input a C++ program and produces a new C++ program that can access its own source code by calling the function kleene::MySource(). This means that you could transform this very simple program into a Quine using the kleene program:

#include <iostream>

int main() {
    std::cout << kleene::MySource() << std::endl;
}

If you’re curious to check it out, it’s available here on my website.

Answer 1

Lots of examples at the Wikipedia article and links therefrom. After looking at one or two it should be obvious how to build a quine generator a given language that takes an arbitrary piece of payload code as input.

One problem with your reflection idea is that the program cannot, in general, know that what it has constructed is its own source code.