Changes

As its name suggests, [[LuaTeX]] adds = Calling Lua, a programming language, to from TeX, the typesetter. This allows us to program TeX in a high-level programming language. For example, consider a TeX macro that divides two numbers. Such a macro is provided by the <tt>fp</tt> package and also by <tt>pgfmath</tt> library of the <tt>TikZ</tt> package. The following comment is from the <tt>fp</tt> package<texcode>\def\FP@div#1#2.#3.#4\relax#5.#6.#7\relax{% % [...] algorithmic idea (for x>0, y>0) % - %determine \FP@shift such that % y*10^\FP@shift < 100000000 % <=y*10^(\FP@shift+1) % - %determine \FP@shift' such that % x*10^\FP@shift'< 100000000 % <=x*10^(\FP@shift+1) % - 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</texcode> The <tt>pgfmath</tt> library implements the macro in a similar way, but limits the number of shifts that it does. These macros highlight the state of affairs in writing TeX macros. Even simple things like multiplying two numbers are hard; you either have to work extremely hard to circumvent the programming limitations of TeX, or, more frequently, hope that someone else has done the hard work for you. In LuaTeX, such a function can be written using the <code>/</code> operator (I will explain the details later):<texcode>\def\DIVIDE#1#2{\directlua{tex.print(#1/#2)}}</texcode>

Thus, with LuaTeX ordinary users can write simple macrosThe interweaving of ConTeXt and Lua consists of two elements: first you tell TeX that you're starting some Lua code; andthen, perhaps more importantlyonce inside Lua, can read and understand macros written by you need to use the appropriate functions to put things into the TeX wizardsstream.

Since the LuaTeX project started it has been actively supported by There are two main ways to execute Lua code in a ConTeXt. These daysdocument: The command <code>\ctxlua</code>, and the various environment <emcode>How do I write such a macro\startluacode...\stopluacode</emcode> questions on the ConTeXt mailing list are answered by a solution that uses Lua. A few such examples Both are presented in this article. This article focuses on how wrappers around the LuaTeX primitive <code>\directlua</code>, which you should never need to use LuaTeX to write macros that require some . In general, you will define a function inside a <emcode>flow control\startluacode</emcode>: randomized outputs, loopsblock, and parsing. For fonts and non-Latin languagesthen define a TeX command that calls the function using <code>\ctxlua</code>, the the [[Fonts in LuaTeX]] pageespecially because <code>\ctxlua</code> has a few idiosyncracies.

= Interaction between The main thing about Lua code in a TeX document is this: the code is expanded by TeX and ''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.

To == Calling 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 bit of <code>\directlua</code>. This Lua instance has a special output stream which can be accessed using <code>tex.print(...)</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>inline: \directlua</code>, the above process is repeated. ctxlua ==

As an exercise, imagine what happens when the following input is processed by LuaTeX. <ref>In this example, two different kinds of quotations are used to avoid escaping quotes. Escaping quotes inside The command <code>\directluactxlua</code> is tricky. The above was a contrived example; if you ever need to escape quotesfor short inline snippets of Lua, you can use the <code>\startluacode ... \stopluacode</code> syntax explained later.</ref>suchas

$2 + 5 \directlua% neq \ctxlua{tex.printcontext("Depth 1 3+5)}$, but is equal to \\directluactxlua{tex.printcontext('Depth 2'+5)}.This is \ctxlua{context(string.upper("absolutely"))}true.

On top of these LuaTeX primitives, ConTeXt provides a higher level interface. There are two ways to call Lua from ConTeXt. The first is a macro <code>\ctxlua</code> (read as ConTeXt Lua), which is similar to <code>\directlua</code>. (Aside: It is possible to run the Lua instance operates under different name spaces. <code>\ctxlua</code> is the default name space; other name spaces are explained later.) <code>\ctxlua</code> is good for calling small snippets of Lua. The argument of <code>\ctxlua</code> is parsed under normal TeX catcodes (category codes), so . This means the end of line character has following two things for the same catcode Lua code inside:* all newlines get treated as a spacespaces* special TeX characters like &, #, $, {, }, etc. This can lead , need to surprisesbe escaped. For example In addition, if you try to use a the warning above still holds. All the Lua commentcode, everything after even the comment gets ignoredcomments, must be valid TeX. Some code to illustrate the newline problem:

This can be avoided by using a TeX comment instead of a Lua comment. However, working under normal TeX catcodes poses a bigger The problem: with special TeX characters like &, #, $, {, }, etc., need to be escaped. For example, # has to be escaped with (<code>\string#t</code> to be used in is Lua for 'the length of array <code>\ctxluat</code>.)

As the argument of <code>== A larger Lua block: \ctxlua</code> is fully expanded, escaping characters can sometimes be trickystartluacode... To circumvent this problem, ConTeXt defines a environment called \stopluacode == Inside the <code>\startluacode ... \stopluacode</code>environment, newlines and special characters behave normally. This sets solves the catcodes to what one would expect in Lua. Basically only catcode problem that <code>\ctxlua</code> has its usual TeX meaningsuffers from. Apart from these special characters, the catcode of everything else is set to other. Somain warning remains in force: all the Lua code, for all practical purposeseven the comments, we can forget about catcodes inside <code>\startluacode ..must be valid TeX. \stopluacode</code>. The above two examples can be written as 

-- A Lua commentThe unknown command \undefined will cause this entire block to fail.  -- Print a countdown '10, 8, ..., 0!' tex -- `..print("This ` is printed.")Lua for string concatenation local t for i = {110,2,3-2 do context(i .. ",4}") tex.print end context("length 0!" )  -- \\par is equivalent to a blank line in the input -- (Notice the escaped backslash: TeX won't mind the above comment.) context. par()  -- Look! we can use # and $ with impunity! context("Unless we print them, then we must \\#t\\$\\& print the escape characters, too.")

The contents of <== Putting Lua code in an external file == You can put your lua code>\startluacodein an external file (with the </code> ... <code>\stopluacodelua</code>, like extension) and include it with the argument of <code>\ctxluarequire</code> are fully expanded. '''This mean that even the Lua comments should be valid TeX statements!''' For example,command: 

-- include the file my-lua-lib.luarequire("my-lua-lib")\undefinedendluacode</texcode> == Namespaces == It is a good habit to put your custom-defined functions in their own namespace. The traditional namespace for this is <code>userdata</code>:<texcode>\startluacode -- if userdata doesn't exist yet, create it userdata = userdata or {} -- define a shorter synonym u = userdata  -- create my custom function inside the userdata namespace function u.myfunction() -- do stuff end

ConTeXt also provides a Lua 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><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 going to be used by other people, you should create your own subnamespaces within these tables. <code><pre>moduledata['mymodule'] = { }mm = moduledata.mymodulefunction to conveniently write to the mm.mainfunction() -- do stuffend</pre></code> = Putting stuff in your TeX streamdocument from Lua = == Simple printing: context(), tex.print(), and tex. The function is called sprint() ==Use <code>context(...)</code> and it for most things. It is equivalent to <code>tex.print(string.format(...))</code>, so <texcode>\startluacodename = "Jane"date = "today"context("Hello %s, how are you %s?", name, date)-- Becomes 'Hello Jane, how are you today?'\stopluacode</texcode> More primitively, you have <code>tex.print()</code> and <code>tex.sprint()</code>. Either one can take as an argument either a number of strings, or an array of strings, and will then insert the strings into the TeX stream. The only difference is that <code>tex.print()</code> treats each string as a separate input line, while <code>tex.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</texcode> but when we use <code>tex.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, escaping arguments can be difficult; instead == Context commands == Most commands that you would type with a backslash in plain ConTeXt, it you can be easier to define a access from Lua function inside 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 context.commands, see the function takes a list [http://www.pragma-ade.com/general/manuals/cld-mkiv.pdf ConTeXt Lua document] manual. It is old, but most of words and chooses a random word among themit still applies. For an example  One final note: arguments can also be specified in the form of such a conversionnested functions. Because LuaTeX evaluates the deepest-nested argument first, see this may cause the <emcode>sorting a list of tokenscontext()</emcode> page calls to be evaluated in the wrong order. For more on this, see the article on [[cld|ConTeXt Lua documents]], and also, again, the [http://Luatexwww.bluwikipragma-ade.com/gogeneral/Sort_a_token_list LuaTeX wikimanuals/cld-mkiv.pdf CLD manual]. = Passing arguments and buffers: ConTeXt commands that hook into Lua = == Making \command{arg1}{arg2} hook into Lua ==First, define a Lua function: <texcode>\startluacode -- remember, using the userdata namespace prevents conflicts userdata = userdata or {}  function userdata.surroundwithdashes(str) context("--" .. str .. "--") end\stopluacode</texcode>

In Then define the above, I created TeX command that expands to a name space called <code>userdata</code> and defined the function <code>random\ctxlua</code> in that name space. Using a name space avoids clashes with the Lua functions defined in LuaTeX and ConTeXt.call:

In order to avoid name clashes, ConTeXt also defines independent name spaces of Lua instances<texcode>\def\surroundwd#1% {\ctxlua{userdata. They are surroundwithdashes([==[#1]==])}}</texcode>

{| |- | '''user'NB'': quoting with <code>[==[#1]==]</code> | a private user instance |-([http://www.lua.org/manual/5.2/manual.html#3.1 long strings]) | '''third'''works just like <code>"#1"</code> in most cases, but in addition | third party module instance it is robust against <code>#1</code> containing the quotation mark |-<code>"</code> which would terminate the Lua string prematurely. | '''module'''Inside <code>\protect .. \unprotect</code> the macros <code>\!!bs</code> | ConTeXt module instanceand <code>\!!es</code> are at your disposition. |They are equivalent to <code>[===[</code> and <code>]===]</code> and -- | '''isolated'''being single tokens to TeX -- parsed faster. | an isolated instance |}(See [http://repo.or.cz/w/context.git/blob/refs/heads/origin:/tex/context/base/luat-ini.mkiv#l174 <code>luat-ini.mkiv</code>].)

Thus, for example, instead of <code>\ctxlua</code> and <code>== Making \startluacode startenv... \stopluacode</code>stopenv hook into Lua ==The first job is, the <code>user</code> instance can be accessed via the macros <code>\usercode</code> and <code>\startusercode ... \stopusercode</code>. In instances other than <code>isolated</code>as ever, all the Lua functions defined by ConTeXt (but not the inbuilt Lua functions) are stored in a <code>global</code> name space. In to have the <code>isolated</code> instance, all Lua functions defined by ConTeXt are hidden and cannot be accessed. Using these instances, we could write function at the above <code>\CHOOSERANDOM</code> macro as followsready

\startusercode math.randomseed( global.os.time() ) function random(...)startluacode global.context(arg[math.random(1, #arg)]) end\stopusercodeuserdata = 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 Lua, simple arithmetic becomes trivial. For example, if you want a macro to find the cosine of an angle (in degrees), you can write

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 setupcolors[state=start] \bTR \bTD $(+)$ \eTD \dorecursesetupTABLE[each][each][width=2em,height=2em,align={6middle,middle} ] {\bTD \recurselevel \eTD} \eTR \dorecurse{6} {\bTR \bTD \recurselevel \eTD setupTABLE[r][1][background=color,backgroundcolor=gray] \edef\firstrecurselevel{\recurselevel} \dorecurse{6} {\bTD \the\numexpr\firstrecurselevel+\recurselevel \eTD}% \eTR} \eTABLE</texcode> However, this does not work as expectedsetupTABLE[c][1][background=color, yielding all zeros.backgroundcolor=gray]

In order to get around the weird rules of macro expansionSo, writing we need a parser in TeX involves function that can take a lot of macro jugglery 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.

\newbox\chemlowbox \def\chemlow#1% {\setbox\chemlowbox \hbox{{\switchtobodyfont[small]#1}}} startluacode

\def\chemhigh#1% -- we will put our molecule function in the userdata namespace. userdata = userdata or {\ifvoid\chemlowbox \high{{\switchtobodyfont[small]#1}}% \else \lohi{\box\chemlowbox} {{\switchtobodyfont[small]#1}} \fi}

\def\finishchem%-- The formatting functions into which the captured {\ifvoid\chemlowbox\else -- superscript/subscript blocks will be fed \lowlocal formatters = {\box\chemlowbox} \fi}

\unexpanded\def\molecule% {\bgroup \catcode`\_=\active \uccode`\~=`\_ \uppercase{\let~\chemlow}% \catcode`\^=\active \uccode`\~=`\^ \uppercase{\let~\chemhigh}% \dostepwiserecurse {65}{90}{1} {\catcode \recurselevel = \active function formatters.low(one) return string.format("\uccode`\~=\recurselevel \uppercaselow{\edef~{\noexpand\finishchem \rawcharacter{\recurselevel}}}}% \catcode`\-=\active \uccode`\~=`\- \uppercase{\def~{--}s}% ", one) \domolecule }% end

function formatters.high(one) return string.format("\def\domolecule#1high{#1\finishchem\egroup%s}", one)</texcode>end

This monstrosity is a typical TeX parserfunction formatters. Appropriate characters need to be made active; occasionallylowhigh(one, <code>two) return string.format("\lccode</code> and <code>\uccode</code> need to be set; signaling tricks are needed (for instancelohi{%s}{%s}", one, checking if <code>\chemlowbox</code> is void); and then magic happens (or so it seems to a flabbergasted usertwo). More sophisticated parsers involve creating finite state automata, which look even more monstrous.end

With LuaTeX, things are differentfunction formatters. LuaTeX includes a general parser based on PEG highlow(parsing expression grammarone, two,three) called [http://www return string.inf.puc-rio.br/roberto/lpeg/lpeg.html lpeg]. This makes writing parsers in TeX much more comprehensible. For exampleformat("\\lohi{%s}{%s}", one, the above <code>\molecule</code> macro can be written astwo)end

<texcode>-- These are the characters we may encounter-- The `/` means we want to expand + and - to \startluacodetextplus c.q. \textminus;-- this substition is not instant, but will take place inside the first -- surrounding lpeg.Cs() call.local plus = lpeg.P("+") / "\\textplus "userdata local minus = userdata or 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("}")

local lowercase = lpeg.R("az")local uppercase = lpeg.R("AZ")local backslash = lpeg.P("\\")local csname = backslash * lpeg.P(1) * (1-backslash)- a ^0or _ affects either a single character, or a brace-delimitedlocal plus = lpeg-- block.P("+") / "\\textplus "local minus = lpegWhichever it is, call it `content`.P("-") / "\\textminus "local digit = lpeg.R("09")local sign single = character + plus + minuslocal cardinal = digit^1local integer = sign^0 * cardinallocal leftbrace = lpeg.P("{")local rightbrace multiple = lpeg.P("}")local nobrace = 1 - (leftbrace + rightbrace)local nested = lpeg.P {leftbrace * (csname + sign + nobrace + lpeg.V(single^1))^0 * rightbrace}local any content = lpeg.P(1)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.PCs("_"content) / formatters.highlocal lowhigh = subscript * lpeg.Cs(content) * superscript = * lpeg.PCs("^"content) / formatters.lowhighlocal somescript highlow = superscript * lpeg.Cs(content) * subscript + superscript* lpeg.Cs(content) / formatters.highlow

-- Finally, the root element: 'moleculepattern'local content moleculepattern = lpeg.Cs(csname (lowhigh + highlow + nested low + sign high + anytext)^0)

local lowhigh = lpegfunction thirddata.Ccmolecule("\\lohi{%s}{%s}"string) * subscript * content * superscript -- * content / `:match` returns the matched string.formatOur patternlocal highlow = lpeg -- `moleculepattern` should match the entire input string.Cc("\\hilo{%s}{%s}") Any * superscript -- * content performed* subscript * content / stringsubstitutions are retained.formatlocal low = lpeg(`.CcCs("\\low{%s}") ` performs a * subscript -- * content / stringpreviously defined substitution.format)local high = lpeg.Cc -- * `context("\\high{%s}") * superscript * content / ` inserts the resulting string.formatinto the stream, ready forlocal justtext -- = (1 - somescript)^1TeX to evaluate.local parser = lpeg.Cscontext(moleculepattern:match(csname + lowhigh + highlow + low + high + sign + anystring)^0)end

function userdata\def\molecule#1{\ctxlua{thirddata.molecule(str"#1") return parser:match(str)end\stopluacode}}

\defstarttext \molecule#1% {Hg^+}, \ctxluamolecule{SO_4^{userdata.molecule("#1")2-}}\stoptext</texcode>

This is more verbose than the TeX solution, but is easier to read Quite terse and write. With a proper readable by parserstandards, I do not have to use tricks to check if either one or both <code>_</code> and <code>^</code> are present. More importantly, anyone (once they know the lpeg syntax) can read the parser and easily understand what isn't it does. This is in contrast to the implementation based on TeX macro jugglery which require you to implement a TeX interpreter in your head to understand.?

== Manipulating verbatim text for dummies ==

Writing macros that manipulate verbatim text involve catcode finesse ''This example demonstrates defining a custom \start...\stop buffer that only TeX wizards can mastergets processed through Lua in its entirety.''

Consider a simple example. Suppose we want to write an environment <code>\startdedentedtyping</code> ... <code>\stopdedentedtyping</code> that removes the indentation of the first line from every line. Thus, the output of ...

Defining an environment in TeX that removes the leading spaces but leaves other spaces untouched is complicated. On the other hand, once we capture the contents of the environment, removing the leading indent or ''dedenting'' the content in Lua is easy. Here is a Lua function that uses lpegsimple stringsubstitutions.

Here is the code for defining the <texcodecode>\unprotect\def\startdedentedtyping% {\begingroup \obeyspaces \obeylines \long\def\dostartdedentedtyping##1\stopdedentedtyping% {\ctxlua{userdata.dedentedtyping(\!!bs \detokenize{##1} \!!es)}% \endgroup}% \dostartdedentedtyping}\protect</texcode>The above macro works for simple cases, but there are some limitations. For example, there is an extra space of <code>.\nstopdedentedtyping</code> in the output. This macro will also fail if the contents have unbalanced braces (try removing the <code>}</code> from the example).pair:

A more robust solution is to use ConTeXt's built in support for buffers. Using buffers, the above macro can be written as <texcode>% Create an environment that stores everything

== Some That's all. Finally, we will go into a little more examples ==detail on how TeX and Lua communicate with each other.

= Conclusion In detail: the interaction between TeX and Lua =

To a first approximation, the interaction between TeX and Lua is straightforward. When TeX (i.e., the LuaTeX is removing many engine) starts, it loads the input file in memory and processes it token by token. When TeX barriers: using system fontsencounters <code>\directlua</code>, it stops reading and writing Unicode filesthe file in memory, typesetting non-Latin languages<em>fully expands the argument of <code>\directlua</code></em>, among othersand passes the control to a Lua instance. HoweverThe Lua instance, which runs with a few preloaded libraries, processes the biggest feature expanded arguments of LuaTeX <code>\directlua</code>. This Lua instance has a special output stream which can be accessed using <code>tex.print(...)</code>. The function <code>tex.print(...)</code> is just like the ability Lua function <code>print(...)</code> except that <code>tex.print(...)</code> prints to use a high-level programming language <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 program TeX. This can potentially lower <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 learning curve for programming <em>TeXstream</em>; and continues. If TeX encounters another <code>\directlua</code>, the above process is repeated.

This article only mentions only one aspect of programming TeX: macros that manipulate their As an exercise, imagine what happens when the following input and output some text to is processed by LuaTeX. The answer is in the main TeX streamfootnotes. Many other <ref>In this example, two different kinds of manipulations quotations are possible: LuaTeX provides access used to avoid escaping quotes. Escaping quotes inside <code>\directlua</code> is tricky. The above was a contrived example; if you ever need to TeX boxesescape quotes, token listsyou can use the <code>\startluacode ... \stopluacode</code> syntax.</ref> <texcode>\directlua% {tex.print("Depth 1 \\directlua{tex.print('Depth 2')}")}</texcode> For more on this, dimensions, glues, catcodes, direction parameters, math parameters, etcsee the [http://wiki.luatex.org/index. The details can be found in php/Writing_Lua_in_TeX] article on the [http://wwwwiki.luatex.org/documentationindex.html php/Main_Page LuaTeX manualwiki].

{{note | This article is a wikified version of originally based on [https://www.tug.org/members/TUGboat/tb30-2/tb95mahajan-luatex.pdf this TugBoat article ]. Feel free to modify it. }}