% engine=luatex language=uk

\startcomponent mk-arabic

\environment mk-environment

\chapter{User code}

Previous versions of \LUATEX\ had multiple \LUA\ instances but in
practice this was not that useful and therefore we decided to
remove that feature and stick to one instance. One reason is that
all activities take place in the zero instance anyway and other
instance could not access variables defined there. Another reason
was that every \type {\directlua} call is in fact a function call
(and as such a closure) and \LUATEX\ catches errors nicely.

The formal \type {\directlua} primitive originally can be called
in two ways:

\starttyping
\directlua <instance> {lua code}
\directlua name {some text} <instance> {lua code}
\stoptyping

The optional text is then part of the error message when one is
issued. The new approach is that the number is used for the error
message in case no \type {name} is specified. The exact string is
set in \LUA. This means that in principle the command is backward
compatible. Old usage will basically ignore the number and use
the one and only instance, while new usage will use the number for
an eventual message:

\starttyping
\directlua <message id> {lua code}
\directlua name {some text} <message id> {lua code}
\stoptyping

In the second case the id is ignored. The advantage of the first
call is that it saves tokens at the \TEX\ end and can be
configured at the \LUA\ end. In \CONTEXT\ \MKIV\ we have adapted
the code that invokes multiple instances by compatible code that
provides a modest form of isolation. We don't want to enforce too
many constraints, first of all because users will often use high
level interfaces anyway, and also because we assume that users have
no bad intentions.

The main \LUA\ instance in \CONTEXT\ is accessible by:

\startbuffer
\startluacode
tex.print("lua")
\stopluacode
\stopbuffer

\typebuffer

This gives: \quote {\getbuffer}.

However, sometimes you don't want user code to interfere too much
with the main code but still provide access to useful data. This
is why we also provide:

\startbuffer
\startusercode
global.tex.print("user 1")
tex.print("user 2")
if characters then
    tex.print("access")
else
    tex.print("no access")
end
tex.print(global.characters.data[0xA9].contextname)
\stopusercode
\stopbuffer

\typebuffer

This gives: \quote {\getbuffer}.

If you're writing a module, you might want to reserve a private
namespace. This is done with:

\startbuffer
\definenamedlua[mymodule][my interesting module]
\stopbuffer

\typebuffer \getbuffer

Now we can say:

\startbuffer
\startmymodulecode
help = { "help" }
tex.print(help[1])
\stopmymodulecode
\stopbuffer

\typebuffer

This gives: \quote {\getbuffer}. The information is remembered:

\startbuffer
\startmymodulecode
tex.print(help[1])
\stopmymodulecode
\stopbuffer

\typebuffer

Indeed we get: \quote {\getbuffer}.

Just to check the isolation we try:

\startbuffer
\startusercode
tex.print(help and help[1] or "no help")
\stopusercode
\stopbuffer

\typebuffer

As expected this gives: \quote {\getbuffer} but when we do the
following we will get an error message:

\startbuffer
\startusercode
tex.print(help[1])
\stopusercode
\stopbuffer

\typebuffer

% {\batchmode \getbuffer} % somehow quits in context but not in texexec

\starttyping
! LuaTeX error <private user instance>:2: attempt to index global
'help' (a nil value)
stack traceback:
	<private user instance>:2: in main chunk.
<inserted text> ...userdata")
tex.print(help[1])
}
\stoptyping

An even more isolated variant is:

\startbuffer
\startisolatedcode
help = { "help" }
tex.print(help and help[1] or "no help")
\stopisolatedcode
\stopbuffer

\typebuffer

We get: \quote {\getbuffer}, while


\startbuffer
\startisolatedcode
tex.print(help and help[1] or "no help")
\stopisolatedcode
\stopbuffer

\typebuffer

gives: \quote {\getbuffer}.

You can get access to the global data of other named code blocks
by using the \type {global} prefix. At that level you have also
access to the instances, but this time we append \type {data}, so
\type {user} has a table \type {userdata}:

\startbuffer
\startmymodulecode
global.userdata.whatever = "be careful!"
\stopmymodulecode
\stopbuffer

For convenience we have made \type {tex} as well as some \LUA\ tables
directly accessible within an instance. However, we recommend not to
extend these yourself (even if we do it in the core of \MKIV).

% not yet ok:
%
% The next example is inspired by a question of Wolfgang Schuster who
% wanted to write a module. Say that we have a file \type {demo.lua}:
%
% \starttyping
% local demo = { }
%
% function demo.whow(str)
%     tex.print("[" .. string.reverse(str) .. "]")
% end
%
% return demo
% \stoptyping
%
% The module can be \type {demo.tex}:
%
% \starttyping
% \definenamedlua[demo][demo module example]
%
% \startdemocode
% demo = global.require("demo.lua")
% \stopdemocode
%
% \def\whow#1{\democode{demo.whow("#1"}}
% \stoptyping
%
% We can now use this module:
%
% \starttyping
% \usemodule[demo]
% \whow{123}
% \stoptyping

\stopcomponent
