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{{note | This is a wikified version of [https://www.tug.org/members/TUGboat/tb30-2/tb95mahajan-luatex.pdf this TugBoat article ]. Feel free to modify it. }}
In this article, I explain how to use Lua to write macros in [[LuaTeX]]. I give some examples of macros that are complicated in [[PdfTeX]], but can be defined easily using Lua in LuaTeX. These examples include macros that do arithmetic on their arguments, use loops, parse their arguments, and manipulate verbatim text.
= Introduction =
Reworded so that it reads like a wiki page
As its name suggests, [[LuaTeX]] adds Lua, a programming language, to 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>
Thus, with LuaTeX ordinary users can write simple macros; and, perhaps more importantly, can read and understand macros written by TeX wizards.
Since the LuaTeX project started it has been actively supported by ConTeXt. <ref>Not surprising, as two of LuaTeX main developers—Taco Hoekwater and Hans Hagen—are also the main ConTeXt developers.</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 A few such examples are presented in this article. I have deliberately avoided examples about [[Fonts in LuaTeX | fonts]] and non-Latin languages. There is already quite a bit of documentation about them. In this This article, I highlight focuses on how to use LuaTeX to write macros that require some <em>flow control</em>: randomized outputs, loops, and parsing. For fonts and non-Latin languages, the the [[Fonts in LuaTeX]] page.
= 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(...)</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.
As an exercise, imagine what happens when the following input is processed by LuaTeX. <ref>In this example, I used 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 explained later.</ref>
<texcode>
= Notes =
<references />
{{note | This is a wikified version of [https://www.tug.org/members/TUGboat/tb30-2/tb95mahajan-luatex.pdf this TugBoat article ]. Feel free to modify it. }}
[[Category:Lua]]
[[Category:LuaTeX]]
[[Category:Programming]]
