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In this article, I explain how to use lua to write macros in [[LuaTeX]]. I give some examples The interweaving of ConTeXt and Lua consists of macros two elements: first you tell TeX that are complicated in [[PdfTeX]]you're starting some Lua code; then, but can be defined easily using lua in luaTeX. These examples include macros that do arithmetic on their argumentsonce inside Lua, you need to use loops, and parse their argumentsthe appropriate functions to put things into the TeX stream.

There are two main ways to execute Lua code in a ConTeXt document: The command <code>\ctxlua</code>, and the environment <code>\startluacode...\stopluacode</code>. Both are wrappers around the LuaTeX primitive <code>\directlua</code>, which you should never need to use. In general, you will define a function inside a <code>\startluacode</code> block, and then define a TeX command that calls the function using <code>\ctxlua</code>, especially because <code>\ctxlua</code> has a few idiosyncracies. The main thing about Lua code in a TeX document is this: the code is expanded by TeX ''before'' Lua gets to it. '''This means that all the Lua code, even the comments, must be valid TeX!''' A string like "\undefined" will cause an immediate failure. = Introduction =Calling a bit of Lua inline: \ctxlua == The command <code>\ctxlua</code> is for short inline snippets of Lua, suchas

\def\FP@div#1#$2.#3.#4+ 5 \relax#5.#6.#7neq \relaxctxlua{% % [...] algorithmic idea context(for x>03+5)}$, y>0) % - %determine but is equal to \FP@shift such that % y*10^\FP@shift < 100000000 % <=y*10^ctxlua{context(\FP@shift2+15)}. % - %determine This is \FP@shift' such that % x*10^\FP@shift'< 100000000 % <=x*10^ctxlua{context(string.upper(\FP@shift+1"absolutely")) % - x=x*\FP@shift' % - y=y*\FP@shift % - \FP@shift=\FP@shift-\FP@shift' % - res=0 % - while y>0 %fixed-point representation! % - \FP@times=0 % - while x>y % - \FP@times=\FP@times+1 % - x=x-y % - end % - y=y/10 % - res=10*res+\FP@times/1000000000 % - end % - %shift the result according to \FP@shift} true.

The <ttcode>pgfmath\ctxlua</ttcode> library implements operates under the macro in a similar way, but limits the number of shifts that it doesnormal TeX catcodes (category codes). These macros highlight This means the state of affairs in writing TeX macros. Even simple following two things like multiplying two numbers are hard; you either have to work extremely hard to circumvent for the programming limitations of Lua code inside:* all newlines get treated as spaces* special TeXcharacters like &, or#, more frequently$, hope that someone else has done the hard work for you{, }, etc., need to be escaped.  In luaTeXaddition, such a function can be written using the <warning above still holds. All the Lua code>/</, even the comments, must be valid TeX. Some code> operator (I will explain to illustrate the details later)newline problem:

\def\DIVIDE#1#2ctxlua {-- A Lua comment tex.print("This is not printed")}\directluactxlua {% A Tex comment tex.print(#1/#2"This is printed")}}

ThusThe problem with special TeX characters. (<code>#t</code> is Lua for 'the length of array <code>t</code>.)<texcode>% This doesn't work:%\ctxlua% {local t = {1,2,3,4}% tex.print("length " .. #t)}\ctxlua {local t = {1, with luaTeX ordinary users can write simple macros; and2, perhaps more importantly3, can read and understand macros written by TeX wizards4} tex.print("length " ..\string#t)}</texcode>

Since the luaTeX project started it has been actively supported by ConTeXt. <ref>Not surprising, as two of luaTeX's main developers\Dash Taco Hoekwater and Hans Hagen== A larger Lua block: \Dash are also the main ConTeXt developersstartluacode.</ref> These days, the various <em>How do I write such a macro</em> questions on the ConTeXt mailing list are answered by a solution that uses lua. I present a few such examples in this article. I have deliberately avoided examples about fonts and non-Latin languages. There is already quite a bit of documentation about them. In this article, I want to highlight how to use luaTeX to write macros that require some <em>flow control</em>: randomized outputs, loops, and parsing.\stopluacode ==

To -- Print a first approximationcountdown '10, 8, the interaction between TeX and lua is straightforward. When TeX (i.e., the luaTeX engine) starts, it loads the input file in memory and processes it token by token0!' -- `. When TeX encounters <code>\directlua</code>, it stops reading the file in memory, <em>fully expands the argument of <code>\directlua</code></em>, and passes the control to a lua instance. The lua instance` is Lua for string concatenation for i = 10, which runs with a few preloaded libraries2, processes the expanded arguments of <code>\directlua</code>. This lua instance has a special output stream which can be accessed using <code>tex.print-2 do context(i ...)</code>. The function <code>tex.print(...", ")</code> is just like the lua function <code>print(...)</code> except that <code>tex.print(...)</code> prints to a <em>TeX stream</em> rather than to the standard output. When the lua instance finishes processing its input, it passes the contents of the <em>TeX stream</em> back to TeX.<ref>The output of <code>tex.print end context(..."0!")</code> is buffered and not passed to TeX until the lua instance has stopped.</ref> TeX then inserts the contents of the <em>TeX stream</em> at the current location of the file that it was reading; expands the contents of the <em>TeX stream</em>; and continues. If TeX encounters another <code>\directlua</code>, the above process is repeated.

As an exercise, imagine what happens when the following input -- \\par is processed by luaTeX. <ref>In this example, I used two different kinds of quotations equivalent to avoid escaping quotes. Escaping quotes inside <code>\directlua</code> is tricky. The above was a contrived example; if you ever need to escape quotes, you can use blank line in the input -- (Notice the escaped backslash: TeX won't mind the <code>\startluacode ..above comment. \stopluacode</code> syntax explained later) context.</ref>par()

<texcode> -- Look! we can use # and $ with impunity!\directlua% {tex.print context("Depth 1 Unless we print them, then we must \\#\\$\\directlua{tex& print the escape characters, too.print('Depth 2')}")}\stopluacode

On top of these luaTeX primitives, ConTeXt provides a higher level interface. There are two ways to call == Putting Lua code in an external file == You can put your lua from ConTeXt. The first is a macro <code>\ctxlua</code> in an external file (read as ConTeXt lua), which is similar to <code>\directluawith the </code>. (Aside: It is possible to run the lua instance under different name spaces. <code>\ctxlua</code> is extension) and include it with the default name space; other name spaces are explained later.) <code>\ctxluarequire</code> is good for calling small snippets of lua. The argument of <code>\ctxlua</code> is parsed under normal TeX catcodes (category codes), so the end of line character has the same catcode as a space. This can lead to surprises. For example, if you try to use a lua comment, everything after the comment gets ignored.command: 

\ctxluastartluacode {-- A include the file my-lua-lib.lua comment tex.printrequire("This is not printedmy-lua-lib")}\endluacode

This can be avoided by using == Namespaces == It is a TeX comment instead of a lua comment. However, working under normal TeX catcodes poses a bigger problem: special TeX characters like \letterampersand, \letterhash, \letterdollar, \{, \}, etc., need good habit to be escapedput your custom-defined functions in their own namespace. For example, \letterhash\ has to be escaped with The traditional namespace for this is <code>\stringuserdata</code> to be used in <code>\ctxlua</code>.:

\ctxluastartluacode {local -- if userdata doesn't exist yet, create it userdata = userdata or {1,2,3,4} tex -- define a shorter synonym u = userdata  -- create my custom function inside the userdata namespace function u.printmyfunction("length " .. ) -- do stuff end \string#t)}stopluacode

As the argument The full list of canonical namespaces, taken from [http://minimals.contextgarden.net/current/context/alpha/tex/context/base/luat-ini.lua luat-ini.lua]: <code>\ctxlua<pre>userdata = userdata or { } -- for users (e.g. functions etc)thirddata = thirddata or { } -- only for third party modulesmoduledata = moduledata or { } -- only for development teamdocumentdata = documentdata or { } -- for users (e.g. raw data)parametersets = parametersets or { } -- experimental for team</pre></code>  If your module, environment, or document is fully expandedgoing to be used by other people, escaping characters can sometimes be trickyyou should create your own subnamespaces within these tables. To circumvent this problem, ConTeXt defines a environment called  <code>\startluacode <pre>moduledata['mymodule'] = { }mm = moduledata.mymodulefunction mm.. \stopluacodemainfunction() -- do stuffend</pre></code> = Putting stuff in your TeX document from Lua = == Simple printing: context(), tex. This sets the catcodes to what one would expect in luaprint(), and tex. Basically only sprint() ==Use <code>\context(...)</code> has its usual TeX meaning, the catcode of everything else for most things. It is set equivalent to other. So, for all practical purposes, we can forget about catcodes inside <code>\startluacode tex.print(string.format(. \stopluacode..))</code>. The above two examples can be written as, so 

-- A lua comment tex.print(name = "This is printed.Jane") local t date = {1,2,3,4}"today" tex.printcontext("length Hello %s, how are you %s?" .. #t, name, date)-- Becomes 'Hello Jane, how are you today?'

This environment is meant for moderately sized More primitively, you have <code>tex.print()</code> and <code snippets>tex. For longer lua sprint()</code>. Either one can take as an argument either a number of strings, it or an array of strings, and will then insert the strings into the TeX stream. The only difference is more convenient to write the that <code>tex.print()</code in > treats each string as a separate lua file and then load it using lua's input line, while <code>dofiletex.sprint()</code> doesn't.So the following lines <texcode>\ctxlua{tex.print("a", "b")}\ctxlua{tex.print({"a", "b"})}</texcode> are both interpreted by TeX as <texcode>ab</codetexcode> function.

ConTeXt also provides a lua function to conveniently write to the TeX stream. The function is called <code>context(...)</code> and it is equivalent to but when we use <code>tex.print(string.format(...))sprint</code>. instead, either of the following

\def\TOHEX#1{\ctxlua{contexttex.sprint("\%Xa",#1"b")}}\TOHEXctxlua{35tex.sprint({"a", "b"})}

The percent sign had to will be escaped because read by TeX as <codetexcode>\ctxluaab</codetexcode> assumes TeX catcodes without any space in between. Sometimes == Context commands == Most commands that you would type with a backslash in plain ConTeXt, escaping arguments you can be difficult; instead, it can be easier to define a lua function inside access from Lua with <code>\startluacode context... \stopluacode</codeem> and call it using command<code/em>\ctxlua</code>. For example, a macro that takes a comma separated list of Unadorned strings and prints a random item can be written end up in TeX as arguments in curly braces; Lua tables end up in TeX as paramater blocks in square brackets. The following two pieces of code are equivalent: 

userdata = userdata or context.chapter({first}, "Some title") math context.randomseedstartcolumns( os{n = 3, rule = "on"}) context("Hello one") context.timecolumn() context("Hello two") function userdata context.randomcolumn(...) context("Hello three") context(arg[math.randomstopcolumns(1, #arg)]) end

<texcode> \CHOOSERANDOMchapter[first]{"Some title} \startcolumns[n=3, rule=on] Hello one", " \column Hello two", " \column Hello three"} \stopcolumns

I could have written For a wrapper so that fuller account of the function takes a list of words and chooses a random word among themcontext. For an example of such a conversioncommands, see the <em>sorting a list of tokens</em> page on the [http://luatexwww.bluwikipragma-ade.com/gogeneral/Sort_a_token_list luaTeX wikimanuals/cld-mkiv.pdf ConTeXt Lua document]manual. It is old, but most of it still applies.

In One final note: arguments can also be specified in the form of nested functions. Because LuaTeX evaluates the abovedeepest-nested argument first, I created a name space called <code>userdata</code> and defined this may cause the function <code>randomcontext()</code> calls to be evaluated in that name spacethe wrong order. Using a name space avoids clashes with For more on this, see the lua functions defined in luaTeX article on [[cld|ConTeXt Lua documents]], and ConTeXtalso, again, the [http://www.pragma-ade.com/general/manuals/cld-mkiv.pdf CLD manual].

In order to avoid name clashes, = Passing arguments and buffers: ConTeXt also defines independent name spaces of lua instances. They are commands that hook into Lua =

== Making \command{| |- | user arg1}{arg2} hook into Lua == | First, define a private user instance |- | third | third party module instance |- | module | ConTeXt module instance |- | isolated | an isolated instance |}Lua function:

\startusercodestartluacode math -- remember, using the userdata namespace prevents conflicts userdata = userdata or {}  function userdata.randomseedsurroundwithdashes( globalstr) context("--" .. str .os.time"--") end\stopluacode</texcode> Then define the TeX command that expands to a <code>\ctxlua</code> call: <texcode>\def\surroundwd#1% {\ctxlua{userdata.surroundwithdashes([==[#1]==]) )}}</texcode> ''NB'': quoting with <code>[==[#1]==]</code> function random([http://www.lua.org/manual/5.2/manual.html#3.1 long strings]) globalworks just like <code>"#1"</code> in most cases, but in addition it is robust against <code>#1</code> containing the quotation mark<code>"</code> which would terminate the Lua string prematurely.Inside <code>\protect .. \unprotect</code> the macros <code>\!!bs</code>and <code>\!!es</code> are at your disposition.They are equivalent to <code>[===[</code> and <code>]===]</code> and --being single tokens to TeX -- parsed faster.context(argSee [mathhttp://repo.or.cz/w/context.git/blob/refs/heads/origin:/tex/context/base/luat-ini.random(1, mkiv#arg)l174 <code>luat-ini.mkiv</code>].) end== Making \startenv...\stopenv hook into Lua ==The first job is, as ever, to have the Lua function at the ready<texcode>\stopusercodestartluacode userdata = userdata or {}

function userdata.verynarrow(buffer) -- equivalent to \def\CHOOSERANDOM#1%startnarrower[10em] context.startnarrower({\usercode{random"10em"}) context(buffer) context.stopnarrower(#1)}} end\stopluacode

Since I defined the function <code>random</code> in the <code>user</code> instance of lua, I did not bother to use a separate name space for the function. The lua functions <code>os.time</code>, which is defined by a luaTeX library, and <code>context</code>, which is defined by ConTeXt, needed to be accessed through a <code>global</code> name space. On the other handNext, we define the <code>math.randomseed</code> function, which is part start command of lua, could be accessed as is. our custom buffer:

! LuaTeX error &lt;main ctx instance&gt;\def\startverynarrow% {\dostartbuffer [verynarrow] % buffer name [startverynarrow] % command where buffer starts [stopverynarrow]} % command where buffer ends % also:command invoked when buffer stops 

With Lastly, we define the <code>\usercodestopverynarrow</code> command such that it passes the error message starts withrecently-complated buffer to our <code>verynarrow</code> Lua function: 

This makes And that's it easier to narrow down the source ! The rest of errorthis article will consist of examples.

Doing simple arithmetic in TeX can be extremely difficult, as illustrated by the division macro in the introduction. With luaLua, simple arithmetic becomes trivial. For example, if you want a macro to find the cosine of an angle (in degrees), you can write

{\ctxlua({context(math.cos(#1*2*math.pi/360))}}

or , if you want less precision (notice the percent sign is escaped):

== Loops without worrying about expansion ==

''This example demonstrates using Lua to write a quasi-repetitive piece of ConTeXt code.'' Loops in TeX are tricky , because macro assignments and macro expansion interact in strange ways. For example, suppose we want to typeset a table showing the sum of the roll of two dice and want the output to look like this:

The tedious (but faster!) way to achieve this This is to simply type easy in LuaTeX. Once a Lua instance starts, TeX does not see anything until the whole table by handLua instance exits.  It is however natural to want to So, we can write this table as a the loopin Lua, and compute simply print the valuesthat we would have typed to the TeX stream. When the control is passed to TeX, TeX sees the input as if we had typed it by hand. A first ConTeXt implementation using This is the Lua code for the recursion level might beabove table:

\bTABLE \bTR \bTD $(+)$ \eTD \dorecurse{6} {\bTD \recurselevel \eTD} setupcolors[state=start] \eTR \dorecursesetupTABLE[each][each][width=2em,height=2em,align={6middle,middle} ] {\bTR setupTABLE[r][1][background=color,backgroundcolor=gray] \bTD \recurselevel \eTD \edef\firstrecurselevel{\recurselevel} \dorecurse{6} {\bTD \the\numexpr\firstrecurselevel+\recurselevel \eTD}% \eTR} \eTABLE</texcode>setupTABLE[c][1][background=color,backgroundcolor=gray]

In order to get around the weird rules of macro expansionAs an example, writing a parser let's consider typesetting chemical molecules in TeX involves . Normally, molecules should be typeset in text mode rather than math mode. If we want :H₄SO₄⁺,we must type :<code>H\low{3}SO\lohi{4}{\textplus}</code>,but we'd much rather type:<code>\molecule{H_3SO_4^+}</code>. So, we need a lot of macro jugglery function that can take a string like that, parse it, and catcode trickeryturn it into the appropriate TeX code. It is LuaTeX includes a black artgeneral parser based on PEG (parsing expression grammar) called [http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html lpeg], one and it makes writing little parsers positively joyful. (Once you've got the knack of it, at least.) For example, the biggest mysteries of TeX for ordinary usersabove <code>\molecule</code> macro can be written as follows.

\newboxstartluacode -- we will put our molecule function in the userdata namespace.userdata = userdata or { } -- The formatting functions into which the captured-- superscript/subscript blocks will be fedlocal formatters = { } function formatters.low(one) return string.format("\chemlowbox \low{%s}", one)end function formatters.high(one) return string.format("\\high{%s}", one)end function formatters.lowhigh(one, two) return string.format("\def\chemlow#1lohi{%s}{% s}", one, two) end function formatters.highlow(one, two,three) return string.format("\\lohi{%s}{%s}", one,two)end -- These are the characters we may encounter-- The `/` means we want to expand + and - to \setboxtextplus c.q. \chemlowboxtextminus; -- this substition is not instant, but will take place inside the first -- surrounding lpeg.Cs() call.local plus = lpeg.P("+") / "\hbox\textplus "local minus = lpeg.P("-") / "\\textminus "local character = lpeg.R("az", "AZ", "09") -- R is for 'range'local subscript = lpeg.P("_") -- P is simply for 'pattern'local superscript = lpeg.P("^")local leftbrace = lpeg.P("{")local rightbrace = lpeg.P("}") -- a ^ or _ affects either a single character, or a brace-delimited-- block. Whichever it is, call it `content`.local single = character + plus + minuslocal multiple = leftbrace * single^1 * rightbracelocal content = single + multiple -- These are our top-level elements: non-special text, of course, and-- blocks of superscript/subscript/both.-- lpeg.Cs(content) does two things:-- (1) not all matches go into the `/ function` construction; only-- *captures* go in. The C in Cs stands for Capture. This way, -- the superscript/subscript mark gets discarded.-- (2) it expands plus/minus before they go into the formatter. The-- s in Cs stands for 'substitute in the replacement values, if any'local text = single^1local low = subscript * lpeg.Cs(content) / formatters.lowlocal high = superscript * lpeg.Cs(content) / formatters.highlocal lowhigh = subscript * lpeg.Cs(content) * superscript * lpeg.Cs(content) / formatters.lowhighlocal highlow = superscript * lpeg.Cs(content) * subscript * lpeg.Cs(content) / formatters.highlow -- Finally, the root element: 'moleculepattern'local moleculepattern = lpeg.Cs((lowhigh + highlow + low + high + text)^0) function thirddata.molecule(string) -- * `:match` returns the matched string. Our pattern -- `moleculepattern` should match the entire input string. Any -- *performed* substitutions are retained. (`.Cs()` performs a -- previously defined substitution.) -- * `context()` inserts the resulting string into the stream, ready for -- TeX to evaluate. context(moleculepattern:match(string))end \stopluacode \def\molecule#1{\switchtobodyfont[small]ctxlua{thirddata.molecule("#1")}} \starttext \molecule{Hg^+} , \molecule{SO_4^{2-}}\stoptext</texcode>

''This example demonstrates defining a custom \unexpandedstart...\def\molecule% {\bgroup \catcode`\_=\active \uccode`\~=`\_ \uppercase{\let~\chemlow}% \catcode`\^=\active \uccode`\~=`\^ \uppercase{\let~\chemhigh}% \dostepwiserecurse {65}{90}{1} {\catcode \recurselevel = \active \uccode`\~=\recurselevel \uppercase{\edef~{\noexpand\finishchem \rawcharacter{\recurselevel}}}}% \catcode`\-=\active \uccode`\~=`\- \uppercase{\def~{--}}% \domolecule }% stop buffer that gets processed through Lua in its entirety.''

Suppose we want to write an environment <code>\defstartdedentedtyping</code> ... <code>\domoleculestopdedentedtyping</code> that removes the indentation of the first line from every line. Thus, the output of ... <texcode>\startdedentedtyping #1include &lt;stdio.h&gt; void main() {#1 print("Hello world \finishchemn") ; }\egroup}stopdedentedtyping

This monstrosity is a typical TeX parser. Appropriate characters need to be made active; occasionally, <codetexcode>\lccode</code> and <code>starttyping#include &lt;stdio.h&gt;void main(){ print("Hello world \uccode</code> need to be setn") ; signaling tricks are needed (for instance, checking if <code>}\chemlowboxstoptyping</codetexcode> is void); and then magic happens (or so it seems to a flabbergasted user)... More sophisticated parsers involve creating finite state automata, which look even more monstrousthough the leading whitespace is different.

With luaTeX, things are different. luaTeX includes a general parser based on PEG (parsing expression grammar) called [http://www.inf.puc-rio.br/roberto/lpeg/lpeg.html lpeg]. This makes writing parsers Defining an environment in TeX much more comprehensiblethat removes the leading spaces but leavesother spaces untouched is complicated. For exampleOn the other hand, once we capture the above <code>\molecule</code> macro can be written ascontents of the environment, removing the leading indent or ''dedenting'' thecontent in Lua is easy. Here is a Lua function that uses simple stringsubstitutions.

-- Initialize a userdata name space to keep our own functions in. -- That way, we won't interfere with anything ConTeXt keeps in -- the global name space. userdata = userdata or {}

local lowercase = lpegfunction userdata.Rdedentedtyping("az"content) local uppercase lines = lpegstring.Rsplitlines("AZ"content) local backslash indent = lpegstring.P("\\")local csname = backslash * lpeg.Pmatch(lines[1) * (1-backslash)], '^0local plus = lpeg.P("+"') / "\\textplus "or '' local minus pattern = lpeg'^' .P("-") / "\\textminus "local digit = lpeg.R("09")indentlocal sign for i= plus + minuslocal cardinal = digit^1,#lines dolocal integer = sign^0 * cardinallocal leftbrace lines[i] = lpegstring.Pgsub(lines[i],pattern,"{")local rightbrace = lpeg.P("}")local nobrace = 1 - (leftbrace + rightbrace)local nested = lpeg.P {leftbrace * (csname + sign + nobrace + lpeg.V(1))^0 * rightbrace}local any = lpeg.P(1)end

local subscript content = lpegtable.Pconcat("_"lines,'\n')local superscript = lpeg.P("^")local somescript = subscript + superscript

local content = lpegtex.Cssprint(csname + nested + sign + any"\\starttyping\n" .. content .. "\\stoptyping\n")

local lowhigh = lpeg.Cc(" -- The typing environment looks for an explicit \type{\lohi{%sstoptyping}{%s}") * subscript * content * superscript * content / string.formatSo,local highlow = lpeg -- context.Ccstarttyping("\\hilo{%s}{%s}") * superscript * context(content ) context.stoptyping() * subscript * content / string -- does not work.formatButlocal low = lpeg -- context.Ccstarttyping("\\low{%s}") * subscript * context(content / string) tex.formatlocal high = lpeg.Ccsprint("\\high{%s}stoptyping") * superscript * content / string -- does.formatlocal justtext = (1 - somescript)^1endlocal parser = lpeg.Cs((csname + lowhigh \stopluacode + highlow + low + high + sign + any)^0)</texcode>

userdataHere is the code for defining the <code>\startdedentedtyping.moleculeparser = parser ..\stopdedentedtyping</code> pair:

<texcode>% Create an environment that stores everything % between \startdedentedtyping and \stopdedentedtyping % in a buffer named 'dedentedtyping'.\def\startdedentedtyping {\dostartbuffer [dedentedtyping] [startdedentedtyping] [stopdedentedtyping]} % On closing the dedentedtyping environment, call the LuaTeX% function dedentedtyping(), and pass it the contents of % the buffer called 'dedentedtyping'\def\stopdedentedtyping {\ctxlua {userdata.moleculededentedtyping(buffers.getcontent(str'dedentedtyping'))}}</texcode> That's all. Finally, we will go into a little more detail on how TeX and Lua communicate with each other.  return parser= In detail:matchthe interaction between TeX and Lua = To a first approximation, the interaction between TeX and Lua is straightforward. When TeX (i.e., the LuaTeX engine) starts, it loads the input file in memory and processes it token by token. When TeX encounters <code>\directlua</code>, it stops reading the file in memory, <em>fully expands the argument of <code>\directlua</code></em>, and passes the control to a Lua instance. The Lua instance, which runs with a few preloaded libraries, processes the expanded arguments of <code>\directlua</code>. This Lua instance has a special output stream which can be accessed using <code>tex.print(str...)</code>. The function <code>tex.print(...)</code> is just like the Lua function <code>print(...)</code> except that <code>tex.print(...)</code> prints to a <em>TeX stream</em> rather than to the standard output. When the Lua instance finishes processing its input, it passes the contents of the <em>TeX stream</em> back to TeX.<ref>The output of <code>tex.print(...)</code> is buffered and not passed to TeX until the Lua instance has stopped.</ref> TeX then inserts the contents of the <em>TeX stream</em> at the current location of the file that it was reading; expands the contents of the <em>TeX stream</em>; and continues. If TeX encounters another <code>\directlua</code>, the above process is repeated. endAs an exercise, imagine what happens when the following input is processed by LuaTeX. The answer is in the footnotes. <ref>In this example, two different kinds of quotations are used to avoid escaping quotes. Escaping quotes inside <code>\directlua</code> is tricky. The above was a contrived example; if you ever need to escape quotes, you can use the <code>\startluacode ... \stopluacode</code> syntax.</ref>

<texcode>\def\molecule#1directlua% {tex.print("Depth 1 \\ctxluadirectlua{userdatatex.moleculeprint("#1'Depth 2')}")}}</texcode>

This is For more verbose than on this, see the TeX solution, but is easier to read and write[http://wiki.luatex.org/index. With a proper parser, I do not have to use tricks to check if either one or both <code>_<php/Writing_Lua_in_TeX] article on the [http:/code> and <code>^</code> are presentwiki. More importantly, anyone (once they know the lpeg syntax) can read the parser and easily understand what it doesluatex.org/index. This is in contrast to the implementation based on TeX macro jugglery which require you to implement a TeX interpreter in your head to understandphp/Main_Page LuaTeX wiki].

= Conclusion Notes =<references />

luaTeX {{note | This article is removing many TeX barriersoriginally based on [https: using system fonts, reading and writing Unicode files, typesetting non//www.tug.org/members/TUGboat/tb30-Latin languages, among others2/tb95mahajan-luatex.pdf this TugBoat article ]. However, the biggest feature of luaTeX is the ability Feel free to use a high-level programming language to program TeX. This can potentially lower the learning curve for programming TeXmodify it.}}

In this article, I have mentioned only one aspect of programming TeX[[Category: macros that manipulate their input and output some text to the main TeX stream. Many other kinds of manipulations are possible: luaTeX provides access to TeX boxes, token lists, dimensions, glues, catcodes, direction parameters, math parameters, etc. The details can be found in the Lua]][[httpCategory://www.luatex.org/documentation.html luaTeX manualLuaTeX]].