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copied!<p>Aside from cheating¹ there is no difference between compiled and interpreted languages. </p> <p>The generic approach to quines is quite easy. First, whatever the program looks like, at some point it has to print something:</p> <pre> print ... </pre> <p>However, what should it print? Itself. So it needs to print the "print" command:</p> <pre> print "print ..." </pre> <p>What should it print next? Well, in the mean time the program grew, so it needs to print the string starting with "print", too:</p> <pre> print "print \"print ...\"" </pre> <p>Now the program grew again, so there's again more to print:</p> <pre> print "print \"print \\\"...\\\"\"" </pre> <p>And so on. With every added code there's more code to print. This approach is getting nowhere, but it reveals an interesting pattern: The string "print \"" is repeated over and over again. It would be nice to put the repeating part into a variable:</p> <pre> a = "print \"" print a </pre> <p>However, the program just changed, so we need to adjust a:</p> <pre> a = "a = ...\nprint a" print a </pre> <p>When we now try to fill in the "...", we run into the same problems as before. Ultimately, we want to write something like this:</p> <pre> a = "a = " + (quoted contents of a) + "\nprint a" print a </pre> <p>But that is not possible, because even if we had such a function <code>quoted()</code> for quoting, there's still the problem that we define <code>a</code> in terms of itself:</p> <pre> a = "a = " + quoted(a) + "\nprint a" print a </pre> <p>So the only thing we can do is putting a place holder into <code>a</code>:</p> <pre> a = "a = @\nprint a" print a </pre> <p>And that's the whole trick! Anything else is now clear. Simply replace the place holder with the quoted contents of <code>a</code>:</p> <pre> a = "a = @\nprint a" print a.replace("@", quoted(a)) </pre> <p>Since we have changed the code, we need to adjust the string:</p> <pre> a = "a = @\nprint a.replace(\"@\", quoted(a))" print a.replace("@", quoted(a)) </pre> <p>And that's it! All quines in all languages work that way (except the cheating ones).</p> <p>Well, you should ensure that you replace only the first occurence of the place holder. And if you use a second place holder, you can avoid needing to quote the string.</p> <p>But those are minor issues and easy to solve. If fact, the realization of <code>quoted()</code> and <code>replace()</code> are the only details in which the various quines really differ.</p> <hr> <p>¹ by making the program read its source file</p>

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