Self-replicating (La)TeX document

Question

Since TeX and LaTeX can print out any text, it should be possible to write a self-replicating document, i.e., a document that is typeset as a PDF/DVI of itself. Have you seen something like that?

Answer 1

A long time ago, in a country far far away, under the influence of Hoefstader's Godel, Escher, Bach, I spent a merry few minutes playing with programs that would print out themselves. One goal was to make a minimal such program in a particular language, another was to have a general scheme that could be added to make any program (in that language) do this (in addition to what the program was supposed to do). In pursuit of that latter goal I figured out some general ingredients that could be used to do this. These were:

1. The ability to convert from an integer to a character.
2. The ability to make decisions.
3. The ability to iterate over a list.

With these, the scheme is as follows. Create a list containing the code converted into some integer representation of the symbols it contains. Insert into that list a special character (usually 0 is a safe bet) at a particular point. Then the program iterates through the list. Its normal behaviour is to convert each integer into the character it represents and output that. However, when it encounters 0 it simply outputs the list.

Here's a TeX version of that:

\tt
\parindent0pt
\emergencystretch3em
\def\A{92, 116, 116, 10, 92, 112, 97, 114, 105, 110, 100, 101, 110, 116, 48, 112, 116, 
10, 92, 101, 109, 101, 114, 103, 101, 110, 99, 121, 115, 116, 114, 101, 116, 99, 104, 
51, 101, 109, 10, 92, 100, 101, 102, 92, 65, 123, 0, 125, 10, 92, 108, 111, 110, 103, 92, 
100, 101, 102, 92, 84, 35, 49, 44, 123, 37, 10, 92, 105, 102, 110, 117, 109, 35, 49, 60, 
48, 92, 114, 101, 108, 97, 120, 10, 92, 101, 108, 115, 101, 10, 92, 105, 102, 110, 117, 
109, 35, 49, 62, 48, 92, 114, 101, 108, 97, 120, 10, 92, 105, 102, 110, 117, 109, 35, 49, 
61, 49, 48, 92, 114, 101, 108, 97, 120, 10, 92, 112, 97, 114, 10, 92, 101, 108, 115, 101, 
10, 92, 99, 104, 97, 114, 35, 49, 10, 92, 102, 105, 10, 92, 101, 108, 115, 101, 10, 92, 
65, 10, 92, 102, 105, 10, 92, 101, 120, 112, 97, 110, 100, 97, 102, 116, 101, 114, 92, 
84, 92, 102, 105, 125, 10, 92, 101, 120, 112, 97, 110, 100, 97, 102, 116, 101, 114, 92, 
84, 92, 65, 92, 98, 121, 101, -1, }
\long\def\T#1,{%
\ifnum#1<0\relax
\else
\ifnum#1>0\relax
\ifnum#1=10\relax
\par
\else
\char#1
\fi
\else
\A
\fi
\expandafter\T\fi}
\expandafter\T\A\bye

This produces:

Answer 2

Save as quine.tex and compile with tex (or pdftex for PDF output):

\def\T{
\tt \hsize 32.5em\parindent 0pt\def \S {\def \S ##1>{}}\S \string
\def \string \T \string {\par \expandafter \S \meaning \T \string
}\par \expandafter \S \meaning \T \footline {} \end }
\tt \hsize 32.5em\parindent 0pt\def \S {\def \S ##1>{}}\S \string
\def \string \T \string {\par \expandafter \S \meaning \T \string
}\par \expandafter \S \meaning \T \footline {} \end

It is due to Péter Szabó and has been published on TUGboat, vol. 29 (2008), p. 207 as part of the TeX Pearls section at EuroBachoTeX 2007.

Here's the output:

Answer 3

Here is a simple example:

\documentclass{article}
\pagestyle{empty}
\usepackage{listings}
\begin{document}
\lstinputlisting{\jobname}
\end{document}

The result looks as the original:

But if you want to be able to copy from the PDF, you must use this code:

\documentclass{article}
\pagestyle{empty}
\usepackage{listings}
\lstset{basicstyle=\ttfamily,flexiblecolumns=true}
\begin{document}
\lstinputlisting{\jobname}
\end{document}

The result:

Answer 4

If inputting the file itself is allowed, here's a shorter version (Plain TeX):

\def\q{\par\begingroup\tt
\obeylines \catcode`\\=12
\catcode`{12\catcode`}=12
\obeyspaces\input\jobname
\endgroup}\q\bye

Alternative version:

\let~=\catcode\def\q{{~`\\12 ~`{12 ~`}12 ~`~12\tt
\obeyspaces\obeylines\input\jobname\relax}}\q\bye

Answer 5

Much impressed and motivated by Andrew Stacey's beautiful answer, I obtained another way to implement his idea about how can go about this. It is a bit different as I use active characters and delimited macros and less of \char although I did use it.

Update: I am adding another shorter method. It is less analogous to Andrew's solution.

LaTeX Update: I am adding a LaTeX solution, in the same spirit. I could have spared a few braces in the \(re)newcommand's but I was already feeling guilty about not using any package... and also about having authorized a \let primitive into the code...

Plain TeX Update: one more incarnation in Plain TeX.

\tt\obeylines\obeyspaces\nopagenumbers
\def\stop{\par\vfill\supereject\end}
\catcode42=13 \catcode60=13 \catcode62=13 
\catcode63=13 \catcode43=13
\chardef<123 \chardef>125 \chardef?35 
\def+{\char92stop}\let*\string
\long\def\a{
*\tt*\obeylines*\obeyspaces*\nopagenumbers
*\def*\stop<*\par*\vfill*\supereject*\end>
*\catcode42=13 *\catcode60=13 *\catcode62=13 
*\catcode63=13 *\catcode43=13
*\chardef*<123 *\chardef*>125 *\chardef*?35 
*\def*+<*\char92stop>*\let***\string
*\long*\def*\a<!>
*\long*\def*\t?1!?2+<?1<*\def**<*\string***\string>
*\def*<<*\string*<>*\def*><*\string*>>
*\def*?<*\string*?>*\def*+<*\string*+>
*\a>?2+>
*\expandafter*\t*\a*\stop}
\long\def\t#1!#2\stop{#1{\def*{\string*\string}
\def<{\string<}\def>{\string>}
\def?{\string?}\def+{\string+}
\a}#2\stop}
\expandafter\t\a\stop

\tt\obeylines\obeyspaces\nopagenumbers
\let~\string \chardef\<=123 \chardef\>=125
\def\4{{\def~{\string~\string}\def\4{\string\4}
\def\<{\string\<}\def\>{\string\>}\a}\>
~\a\par\vfill\eject\end}
\def\a{
~\tt~\obeylines~\obeyspaces~\nopagenumbers
~\let~~~\string ~\chardef~\<=123 ~\chardef~\>=125
~\def~\4\<\<~\def~~\<~\string~~~\string\>~\def~\4\<~\string~\4\>
~\def~\<\<~\string~\<\>~\def~\>\<~\string~\>\>~\a\>~\>
~~~\a~\par~\vfill~\eject~\end\>
~\def~\a\<
\4}
\a

\documentclass{article}
\usepackage[T1]{fontenc}
\begin{document}
\obeylines\thispagestyle{empty}
\let~\textbackslash
\newcommand{\asciitilde}{\raisebox{-.5\height}{\textasciitilde}}
\newcommand{\mymyself}{{\let~\asciitilde
\renewcommand{\{}{\textbackslash\textbraceleft}
\renewcommand{\}}{\textbackslash\textbraceright}
\renewcommand{\asciitilde}{\textbackslash asciitilde}
\renewcommand{\mymyself}{\textbackslash mymyself}\myself}}
\newcommand{\myself}{
~documentclass\{article\}
~usepackage[T1]\{fontenc\}
~begin\{document\}
~obeylines~thispagestyle\{empty\}
~let\asciitilde~textbackslash
~newcommand\{~asciitilde\}\{~raisebox\{-.5~height\}\{~textasciitilde\}\}
~newcommand\{~mymyself\}\{\{~let\asciitilde~asciitilde
~renewcommand\{~\{\}\{~textbackslash~textbraceleft\}
~renewcommand\{~\}\}\{~textbackslash~textbraceright\}
~renewcommand\{~asciitilde\}\{~textbackslash asciitilde\}
~renewcommand\{~mymyself\}\{~textbackslash mymyself\}~myself\}\}
~newcommand\{~myself\}\{\mymyself\}
~myself
~end\{document\}}
\myself
\end{document}

\tt\obeylines
\nopagenumbers\let~\string
\def\a{~\tt~\obeylines
~\nopagenumbers~\let~~~\string
~\def~\a~{\b~}
~\def~\b~{~{~\def~~~{~\string~~~\string~}~\def~\b~{~\string~\b~}~\a~}~}
~\a~\b~ye}
\def\b{{\def~{\string~\string}\def\b{\string\b}\a}}
\a\bye