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I think the questions says it all! :)

By TeX, I mean any derivative of TeX as long as the code is clearly a derivative of TeX, i.e. that it could belong on TeX.SX (and not pure Lua for example).

I'll give two answers myself so you see what I mean.

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6  
Fully-related: What can't TeX do? –  Werner Mar 25 '13 at 18:55
12  
We tried to elect the Pope. Maybe next time. :) –  Paulo Cereda Mar 25 '13 at 19:05
3  
Related, or somewhat similar: tex.stackexchange.com/questions/53082/… –  zeroth Mar 25 '13 at 19:31
3  
Related:Self-replicating (La)TeX document –  texenthusiast Mar 25 '13 at 20:43
4  

14 Answers 14

up vote 79 down vote accepted

I think that Steve Hicks controller for a Mars Rover programmed in TeX is a good candidate: ICFP Contest 2008 - Mars rover in TeX.

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2  
The writeup for this one is epic, as such tales usually are. –  Ryan Reich Mar 25 '13 at 20:09

David Carlisle's Christmas 'card' at http://www.ctan.org/pkg/xii. In fact, here it is: run this through plain TeX:

\let~\catcode~`76~`A13~`F1~`j00~`P2jdefA71F~`7113jdefPALLF
PA''FwPA;;FPAZZFLaLPA//71F71iPAHHFLPAzzFenPASSFthP;A$$FevP
A@@FfPARR717273F737271P;ADDFRgniPAWW71FPATTFvePA**FstRsamP
AGGFRruoPAqq71.72.F717271PAYY7172F727171PA??Fi*LmPA&&71jfi
Fjfi71PAVVFjbigskipRPWGAUU71727374 75,76Fjpar71727375Djifx
:76jelse&U76jfiPLAKK7172F71l7271PAXX71FVLnOSeL71SLRyadR@oL
RrhC?yLRurtKFeLPFovPgaTLtReRomL;PABB71 72,73:Fjif.73.jelse
B73:jfiXF71PU71 72,73:PWs;AMM71F71diPAJJFRdriPAQQFRsreLPAI
I71Fo71dPA!!FRgiePBt'el@ lTLqdrYmu.Q.,Ke;vz vzLqpip.Q.,tz;
;Lql.IrsZ.eap,qn.i. i.eLlMaesLdRcna,;!;h htLqm.MRasZ.ilk,%
s$;z zLqs'.ansZ.Ymi,/sx ;LYegseZRyal,@i;@ TLRlogdLrDsW,@;G
LcYlaDLbJsW,SWXJW ree @rzchLhzsW,;WERcesInW qt.'oL.Rtrul;e
doTsW,Wk;Rri@stW aHAHHFndZPpqar.tridgeLinZpe.LtYer.W,:jbye

A good collection of Enjoy TeX pearls diving! at GUST, Polish TeX Users Group

For more Pearls of TeX programming at TUGboat ,Volume 26 (2005), No. 3.

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4  
Could anyone please explain how it works? –  Uwe Ziegenhagen Mar 25 '13 at 20:11
10  
@percusse I think Uwe meant how common mortals are supposed to understand David's code. I know: we're not :) –  Xavier Mar 25 '13 at 20:17
9  
@UweZiegenhagen It's just a typical plain TeX file, the syntax is slightly different to the LaTeX syntax that's more commonly seen here. –  David Carlisle Mar 25 '13 at 20:55
9  
If your French is better than mine... groups.google.com/forum/?hl=fr&fromgroups=#!topic/… –  David Carlisle Mar 25 '13 at 21:21
2  
For something similar, and a detailed explanation of the code, see Can you explain how this code works? –  Werner Mar 29 '13 at 19:53

Adding coffee stains to your documents

If my documents don't have those stains, my boss / students think(s) I don't drink coffee. If he or they think I am not drinking coffee, they believe I am slacking. Thanks Hanno!

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6  
Great! I need it. I didn't find the package on CTAN - would be nice, if it would be part of TeXLive. –  knut Mar 25 '13 at 20:05

a basic interpreter written in tex.

see the tugboat article.

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1  
Does it support Commodore Basic? My Dad could run the program he typed up on the C64 to play Star Fleet battles on it! –  Canageek Mar 25 '13 at 18:59

Our own Bruno LeFloch who wrote a Reversi game which runs in the console:

(Please don't try to reformat the code displayed below unless you really know what you are doing; if you do attempt a reformat, try to compile the resulting code before replacing the code here.)

% !TEX TS-program = tex
\long\def\3#1#2#3{}\vsize5cm\hsize4cm\newlinechar`*\def~#1{\catcode`#113~}
~QSU_VWJKLMNO@XY[](|+Z'"z:qj^;/)!,\ ${*13\3}
\def~#1#2{\let#1#2~}~*\cr[\ifnum(\ifcaseO\or|\else]\fiN\number@\advanceX
\expandafterZ\globalY\message~\defj{[0<Q[9>Q[0<J[9>J^|_]|_]|_]|_]}
~+{\count1}+1=9~_#1{@+1 1\countdef#1+1_}_QJVSKWUL,'"$\H!_-1'1"2+44'+55'+45"+54"~^{+NQNJ}
~:#1{#11#12#13#14#15#16#17#18}
~M#1{Y{#1}#1}~\h#1#2{M#2:{ \q#1}&M#2&M{*}}~\q#1#2{&M{(+#1#2  O-O0]}}
~/{Y{Row and column? e.g. E6*}\read_to\MX\j\meaning\M  ;}
~\j#1->#2#3#4;{Q`#2@Q-`@J`#3@J-`0;(VY{Invalid move.}
/]}~;{V0 (jS1z1z0z_S0z1z_S_z1z0z_]}~\_{@,('O-]}
~z#1{{\H0K#1!1{\H1q}(!q]}}~q{@QS@JK[j="(\HZ^'Z\_2]\&q|[j='ZVV($(\H|Z!0]]]]}~\,#1{Q#1:\.}
~\.#1{J#1;[0<V\&[V>WWVUQLJ]]}~\^#1{(#1O0O1O2O2O2O2O1O0]}
~\&{!\^Q\multiply!3@!\^J@V(!9O1O6O1O1O2O6O2O4] }~\Z{M :{&M}&M{*}}
~){'X"X"N'\halign{&## *M{*}
\Z\h1A\h2B\h3C\h4D\h5E\h6F\h7G\h8H\Z}
\vfil\break$1W(W_|0] :\,$0 [0<W[1='QUJL|/];^'\_1][_=WM
{(,Tie| Player [0>,-|0] wins by N[0>,-],].}X\end])})
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It is – intentionally, of course – named reverxii in reminescence to David Carlisle. Just compare Peter Flynn’s answer here. –  Speravir Mar 26 '13 at 16:47
1  
I don't know why it's taken me so long to run this through... this is absolutely insane. Why? Why? –  Sean Allred Dec 30 '13 at 12:45

I once spent hours learning enough TeX to format my ex-gf's resume for printing on the computing center laser printer (back when laser printing was magical) and used up most of my monthly laser printing quota printing copies of it -- all under the mistaken belief that she'd see that she was crazy to break up with me.

Not surprisingly, it turned out to be a useless waste of my time (and in retrospect, it was me that should have broken up with her). Worse, word got around that I was a "TeX expert" and I ended up spending the rest of my computer center operator job helping grad students format their theses

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4  
Sorry, but this not an answer to the spirt of the question. –  Speravir Mar 26 '13 at 17:26
6  
Maybe I misunderstood the {fun} tag –  Johnny Mar 26 '13 at 17:40
28  
@Speravir: I'm not sure about the "spirit of the question", but for me, using TeX for making a gf regret breaking up definitely counts. –  mbork Mar 26 '13 at 18:57
43  
Well, I actually once met a nice girl because she needed help formatting her thesis and was told I was a "TeX expert". She's now my wife, and we had our first child 6 months ago :) –  Xavier Mar 26 '13 at 21:12
2  
@Xavier: congratulations! –  mbork Mar 26 '13 at 23:26

Solving a non-linear equation

Not typesetting the solution (actually, also typesetting the solution of course :)), but more bizarrely implementing the bisection and secant non-linear solvers in TeX!

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You know, I am at this moment wondering whether there is an easy way to graph some trajectories of a nonlinear system of ODEs in pgfplots. Obviously, my search is ended. (Oh, this only does algebraic equations. Boooo!) –  Ryan Reich Mar 25 '13 at 20:11
    
@RyanReich: You may have a look at ctan.org/tex-archive/graphics/pstricks/contrib/pst-ode . Though I must admit that the actual calculation is done by your Postscript printer rather than by TeX. –  AlexG May 21 '13 at 11:30

This one is probably my best:

But there are a few of them scattered around on this site. Here are my picks:

... and, as a bonus:

I really don't see why someone would go back to MS Office after using LaTeX... Now that's weird! ;)

EDIT: and i just remembered this one (Why facebook implemented it? But why recreate it in LaTeX? ;)):

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I am the originator of the "How should I convert my slides to PowerPoint...?" question and if you read it you will note that it was not my desire to back to MS Office. But I'm glad the question has achieved a measure of notoriety. –  Matthew Leingang Mar 27 '13 at 2:16
    
@MatthewLeingang: I know you were forced into it. Nevertheless, it was a weird thing to do IMHO and I'm only happy to advertise it. :) –  Count Zero Mar 29 '13 at 21:49

Well, as made famous by a question of mine I have used LaTeX to make to make props for a Call of Cthulhu game. I also posted the finished product on my blog.

Then later on I used the same technique to make my teaching assistant's life a bit more interesting

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Calculating π with TeX

Gen­er­ates π, us­ing the for­mula

pi=16*arc­tan(1/5)-4*arc­tan(1/239)

and leaves the re­sult in an ar­ray \xr, print­ing what it’s cal­cu­lated as it goes along.

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Implementing the bisection method (and other numerical methods) in TikZ,
for exposition purposes.

EDIT: Great thanks to percusse for helping me improve my \ifg command.

enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here enter image description here

\documentclass[dvipsnames]{beamer}

\usepackage{lmodern}
\usepackage{pgfplots}

\usefonttheme[onlymath]{serif}
\setbeamertemplate{navigation symbols}{}

% 'if #1 greater than #2 then #3 else #4' construct (compatible with pgfmath)
\newcommand{\ifg}[4]{
    \pgfmathparse{(#1)>(#2)?int(1):int(0)}
    \ifnum\pgfmathresult=1relax%
        #3%
    \else%
        #4%
    \fi%
}

\begin{document}

\begin{frame}[fragile]
    \begin{center}
    \begin{tikzpicture}[scale=6]
    \pgfmathsetmacro{\extendxaxis}{.1}
    \pgfmathsetmacro{\extendyaxis}{.3}
    \colorlet{acolor}{red}
    \colorlet{bcolor}{OliveGreen}
    \colorlet{ccolor}{orange}
    \colorlet{fcolor}{blue}
    \pgfmathdeclarefunction{f}{1}{\pgfmathparse{.5*(exp(-#1)-#1)}} % continuous function
    \pgfmathsetmacro{\ao}{.2}   % lower-bound of initial bracket
    \pgfmathsetmacro{\bo}{1}    % upper-bound of initial bracket
    \pgfmathsetmacro{\co}{0}    % midpoint of inital bracket (initialised at 0)
    \pgfmathtruncatemacro{\n}{3}        % number of iterations

    \ifx\ao\bo % check that \ao and \bo are distinct 
        \node[red] (0,0) {Bisection impossible: singleton initial bracket.};
    \else%
        \ifg{\ao}{\bo}{ % if \ao > \bo, swop them
            \pgfmathsetmacro{\temp}{\bo}
            \pgfmathsetmacro{\bo}{\ao}
            \pgfmathsetmacro{\ao}{\temp}
        }{}
        % code in here
        \ifg{{f(\ao)*f(\bo)}}{0}{%
            \node[red] (0,0) {Bisection impossible: no root in initial bracket.};
        }{%
            \onslide<1->{%
                \pgfmathsetmacro{\xmin}{\ao-.5*\extendxaxis*(\bo-\ao)}
                \pgfmathsetmacro{\xmax}{\bo+.5*\extendxaxis*(\bo-\ao)}
                \draw[->] (\xmin,0) node[left] {$0$} --
                    (\xmax,0) node[right] {$x$};
                \pgfmathsetmacro{\ymin}{min(f(\ao),f(\bo))-.5*\extendyaxis*abs(f(\bo)-f(\ao))}
                \pgfmathsetmacro{\ymax}{max(f(\ao),f(\bo))+.5*\extendyaxis*abs(f(\bo)-f(\ao))}
                \draw[->] (\xmin,\ymin) -- (\xmin,\ymax) node[left] {$f(x)$};
                \draw[fcolor] plot[domain=\xmin:\xmax] (\x,{f(\x)});
            }
            \pgfplotsforeachungrouped \k in {1,2,...,\n}{%
                \pgfmathtruncatemacro{\k}{\k}
                \pgfmathsetmacro{\co}{0.5*(\ao+\bo)}
                \pgfmathtruncatemacro{\j}{\k-1}
                \pgfmathtruncatemacro{\fromslide}{3*(\k-1)+2}
                \pgfmathtruncatemacro{\toslide}{\fromslide+2}
                \onslide<\fromslide-\toslide>{
                    \draw[thick,acolor] (\ao,.1ex) --
                        (\ao,-.1ex) node[below] {$a_{\j}$}; % a0
                    \draw[thick,bcolor] (\bo,.1ex) --
                        (\bo,-.1ex) node[below] {$b_{\j}$}; % b0
                }
                \pgfmathtruncatemacro{\fromslide}{\fromslide+1}
                \onslide<\fromslide-\toslide>{%
                    \draw[thick,ccolor] (\co,.1ex) --
                        (\co,-.1ex) node[below] {$c_{\j}$};     
                }
                \onslide<\toslide>{%
                    \coordinate (a0) at (\ao,{f(\ao)});     
                    \draw[dashed,thin,red]  (a0 |- \xmin,0) -- (a0)
                        -- (a0 -| \xmin,0) node[left] {$f(a_{\j})$};% dashed lines
                    \draw[fill,red] (a0) circle (.05ex);            
                    \coordinate (c0) at (\co,{f(\co)}); 
                    \draw[dashed,thin,orange]   (c0 |- \xmin,0) -- (c0)
                        -- (c0 -| \xmin,0) node[left] {$f(c_{\j})$};% dashed lines      
                    \draw[fill,orange] (c0) circle (.05ex);                                 
                }
                \ifg{f(\ao)*f(\co)}{0}{ % bisection iteration
                    \pgfmathsetmacro{\ao}{\co}
                }{%
                    \pgfmathsetmacro{\bo}{\co}
                }
                \ifx\n\k%
                    \pgfmathtruncatemacro{\toslide}{\toslide+1}
                    \pgfmathtruncatemacro{\j}{\j+1}
                    \onslide<\toslide>{%
                        \draw[thick,acolor] (\ao,.1ex) --
                            (\ao,-.1ex) node[below] {$a_{\j}$}; % a0 
                        \draw[thick,bcolor] (\bo,.1ex) --
                            (\bo,-.1ex) node[below] {$b_{\j}$}; % b0 
                    }
                \fi                     
            }
        }
    \fi
    \end{tikzpicture}
    \end{center}
\end{frame}
\end{document}
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Do I need to do anything special to compile your code? It hangs with ` File ended while scanning use of \next.` :( –  Xavier Mar 25 '13 at 19:28
    
Weird. Works great on my machine, but copying and pasting it on the site introduces an error somewhere... I'll figure it out. –  Jubobs Mar 25 '13 at 19:35
2  
@Xavier Remove the white-space in front of \end{frame} and it works. The manual states for using fragile: “In this case, […] the \end{frame} must be alone on a single line.” –  Qrrbrbirlbel Mar 25 '13 at 19:56
    
@Qrrbrbirlbel Thanks! Should work now. –  Jubobs Mar 25 '13 at 20:00
    
@Qrrbrbirlbel Thanks! If TeX starts to be picky about spaces, I am lost :) –  Xavier Mar 25 '13 at 20:00

Implementing a Turing machine simulator
(TeX is Turing complete)

Hail to the busy beaver!

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Providing an excuse for slacking off

Just let (La)TeX burn some CPU cycles while you are relaxing.

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1  
The #1 programmers excuse for legitimately slacking off: xkcd.com/303 –  moose Jul 10 '13 at 17:18

Enrico “egreg” Gregorio posted this into our TeX.sx chat:

  1. Let me name it xcix.tex, because it’s in the manner of David Carlisle’s xii.tex, cf. Peter Flynn’s answer

    \let~\catcode~`x13~`q~`x~`z~`q~`H~`q~`B~`H~`j0~`jA009
    jlet~jlet~Hjpar ~Bjmscount~~jdef~x{q bottlez of beer}
    ~jw{x on the wall}~jt{jadvanceAB-1ATake one down Aand
    pass it around,H}B99~ji{jifnumB}~q{ji=0Nojelsejnumber
    Bjfi}~z{ji>1sjfiA }jloop jifnumB>0 jw, x,Hjtjw.jvskip
    8ptplus1ptjrepeat Time to buy some more beerjdotsjend
    

    (source link)

  2. The same as LaTeX3 version

    \documentclass{article}
    \usepackage{xparse}
    
    \setlength{\parindent}{0pt}
    \setlength{\parskip}{1.5ex}
    
    \ExplSyntaxOn
    % user level command
    \NewDocumentCommand{\beers} { O{99} }
     {
      \manual_beers_sing:n { #1 }
     }
    % variables
    \int_new:N \l_manual_beers_count_int
    % functions
    \cs_new_protected:Npn \manual_beers_sing:n #1
     {
      \int_set:Nn \l_manual_beers_count_int { #1 }
      \prg_replicate:nn { \l_manual_beers_count_int }
       {
        \manual_beers_print:
        \int_decr:N \l_manual_beers_count_int
       }
      Time ~ to ~ buy ~ some ~ more ~ beer ~ \dots
     }
    \cs_new_protected:Npn \manual_beers_print:
     {
      \manual_beer_text:nn { ~ on ~ the ~ wall }{ 0 }, ~
      \manual_beer_text:nn { } { 0 }, \\
      Take ~ one ~ down ~ and ~ pass ~ it ~ around, \\
      \manual_beer_text:nn { ~ on ~ the ~ wall } { -1 }.\par
     }
    \cs_new:Npn \manual_beer_text:nn #1 #2
     {
      \int_case:nnF { \l_manual_beers_count_int + #2 }
       {
        { 0 } { No ~ bottle ~ of ~ beer }
        { 1 } { 1 ~ bottle ~ of ~ beer }
       }
       {
        \int_to_arabic:n { \l_manual_beers_count_int + #2 } ~ bottles ~ of ~ beer
       }
      #1
     }
    \ExplSyntaxOff
    \begin{document}
    \beers
    \end{document}
    

    (source link, small correction; the code as seen above is, though, in this version, as it will appear in a manual by Enrico, which is in the moment, this code was posted, in process of writing)

This was influenced by me, because I had posted these two links:

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protected by Loop Space Mar 26 '13 at 23:05

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