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There are two main ways to execute Lua code in a ConTeXt document: The command <code>\{{cmd|ctxlua</code>}}, and the environment <code>\{{cmd|startluacode...\}}…{{cmd|stopluacode</code>}}. Both are wrappers around the LuaTeX primitive <code>\{{cmd|directlua</code>}}, which you should never need to use. In general, you will define a function inside a <code>\{{cmd|startluacode</code> }} block, and then define a TeX command that calls the function using <code>\{{cmd|ctxlua</code>, especially because <code>\{{cmd|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 "\{{cmd|undefined" }} will cause an immediate failure.

== Calling a bit of Lua inline: \{{cmd|ctxlua }} ==

The command <code>\{{cmd|ctxlua</code> }} is for short inline snippets of Lua, such

<code>\{{cmd|ctxlua</code> }} operates under the normal TeX catcodes (category codes). This means the following two things for the Lua code inside:

== Calling a lua function with \{{cmd|cldcontext }} and get the return ==

One can execute a Lua code from within TeX and get back the result in TeX by using {{codecmd|\cldcontext}}. Thus, if {{code|myfunction}} is a function of a variable {{code|x}} defined in Lua, {{codecmd|\cldcontext|{myfunction(5)}}} returns the value {{code|myfunction(5)}} in TeX. This is equivalent to {{codecmd|\ctxlua|{context(myfunction(5))}}}.

== A larger Lua block: \{{cmd|startluacode...\}}…{{cmd|stopluacode }} ==

Inside the <code>\{{cmd|startluacode...\}}…{{cmd|stopluacode</code> }} environment, newlines and special characters behave normally. This solves the catcode problem that <code>\{{cmd|ctxlua</code> }} suffers from. Apart from these special characters, the main warning remains in force: all the Lua code, even the comments, must be valid TeX.

-- Print a countdown '10, 8, ...…, 0!'

end

Use <code>context(...…)</code> for most things. It is equivalent to <code>tex.print(string.format(...…))</code>, so

Then define the TeX command that expands to a <code>\{{cmd|ctxlua</code> }} call:

Inside <code>\{{cmd|protect .. \}}…{{cmd|unprotect</code> }} the macros <code>\{{cmd|!!bs</code>}}and <code>\{{cmd|!!es</code> } are at your disposition.

== Making \{{cmd|startenv...\}}…{{cmd|stopenv }} hook into Lua ==

Lastly, we define the <code>\{{cmd|stopverynarrow</code> }} command such that it passes the recently-complated buffer to our <code>verynarrow</code> Lua function:

:<code>H\{{cmd|low|{3}}}SO\{{cmd|lohi|{4}}}{{{\cmd|textplus}}}</code>,

:<code>\{{cmd|molecule|{H_3SO_4^+}</code>}}.

So, we need a function that can take a string like that, parse it, and turn it into the appropriate TeX code. LuaTeX includes a general parser based on PEG (parsing expression grammar) called [http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html lpeg], and it makes writing little parsers positively joyful. (Once you've got the knack of it, at least.) For example, the above <code>\{{cmd|molecule</code> }} macro can be written as follows.

''This example demonstrates defining a custom \{{cmd|start...\}}…{{cmd|stop }} buffer that gets processed through Lua in its entirety.''

Suppose we want to write an environment <code>\{{cmd|startdedentedtyping</code> ... <code>\}} … {{cmd|stopdedentedtyping</code> }} that removes the indentation of the first line from every line. Thus, the output of ...…

... … should be the same as the output of ...…

... … even though the leading whitespace is different.

Here is the code for defining the <code>\{{cmd|startdedentedtyping...\}} … {{cmd|stopdedentedtyping</code> }} pair:

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>\{{cmd|directlua</code>}}, it stops reading the file in memory, <em>fully expands the argument of <code>\{{cmd|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>\{{cmd|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>\{{cmd|directlua</code>{{cmd|, the above process is repeated.

As 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>\{{cmd|directlua</code> }} is tricky. The above was a contrived example; if you ever need to escape quotes, you can use the <code>\{{cmd|startluacode ... }}…{{cmd|\stopluacode</code> {{cmd| syntax.</ref>