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% language=uk
\startcomponent mk-dirtytricks
\environment mk-environment
\chapter {Dirty tricks}
If you ever laid your hands on the \TeX book, the words \quote {dirty tricks} will
forever be associated with an appendix of that book. There is no doubt that you need
to know a bit of the internals of \TEX\ in order to master this kind of trickyness.
In this chaper I will show a few dirty \LUATEX\ tricks. It also gives an impression
of what kind of discussions Taco and I had when discussing what kind of support should
be build in the interface.
\subject{afterlua}
When we look at \LUA\ from the \TEX\ end, we can do things like:
\startbuffer
\def\test#1{%
\setbox0=\hbox{\directlua0{tex.sprint(math.pi*#1)}}%
pi: \the\wd0\space\the\ht0\space\the\dp0\par
}
\stopbuffer
\typebuffer \blank \getbuffer \blank
But what if we are at the \LUA\ end and want to let \TEX\ handle things? Imagine
the following call:
\startbuffer
\setbox0\hbox{} \dimen0=0pt \ctxlua {
tex.sprint("\string\\setbox0=\string\\hbox{123}")
tex.sprint("\string\\the\string\\wd0")
}
\stopbuffer
\typebuffer
This gives: \ignorespaces \getbuffer. This may give you the impression that \TEX\
kicks in immediately, but the following example demonstrates otherwise:
\startbuffer
\setbox0\hbox{} \dimen0=0pt \ctxlua {
tex.sprint("\string\\setbox0=\string\\hbox{123}")
tex.dimen[0] = tex.box[0].width
tex.sprint("\string\\the\string\\dimen0")
}
\stopbuffer
\typebuffer
This gives: \getbuffer. When still in \LUA, we never get to see the width
of the box.
A way out of this is the following rather straightforward approach:
\starttyping
function test(n)
function follow_up()
tex.sprint(tex.box[0].width)
end
tex.sprint("\\setbox0=\\hbox{123}\\directlua 0 {follow_up()}")
end
\stoptyping
We can provide a more convenient solution for this:
\starttyping
after_lua = { } -- could also be done with closures
function the_afterlua(...)
for _, fun in ipairs(after_lua) do
fun(...)
end
after_lua = { }
end
function afterlua(f)
after_lua[#after_lua+1] = f
end
function theafterlua(...)
tex.sprint("\\directlua 0 {the_afterlua("
.. table.concat({...},',') .. ")}")
end
\stoptyping
If you look closely, you will see that we can (optionally) pass arguments
to the function \type {theafterlua}. Usage now becomes:
\starttyping
function test(n)
afterlua(function(...)
tex.sprint(string.format("pi: %s %s %s\\par",... ))
end)
afterlua(function(wd,ht,dp)
tex.sprint(string.format("ip: %s %s %s\\par",dp,ht,wd))
end)
tex.sprint(string.format("\\setbox0=\\hbox{%s}",math.pi*n))
local box_0 = tex.box[0]
theafterlua(box_0.width,box_0.height,box_0.depth)
end
\stoptyping
The last call may confuse you but since it does a print to \TEX, it is
in fact a delayed action. A cleaner implementation is the following:
\starttyping
local delayed = { }
local function flushdelayed(...)
delayed = { }
for i=1, #t do
t[i](...)
end
end
function lua.delay(f)
delayed[#delayed+1] = f
end
function lua.flush(...)
tex.sprint("\\directlua{flushdelayed(" ..
table.concat({...},',') .. ")}")
end
\stoptyping
Usage is similar:
\starttyping
function test(n)
lua.delay(function(...)
tex.sprint(string.format("pi: %s %s %s\\par",...))
end)
tex.sprint(string.format("\\setbox0=\\hbox{%s}",math.pi*n))
local box_0 = tex.box[0]
lua.flush(box_0.width,box_0.height,box_0.depth)
end
\stoptyping
\stopcomponent
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