System Macros/Fundamentals
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The ConTeXt version
\contextversion
contains the ConTeXt version string. If you need to make sure you are running under ConTeXt, check for this macro. It is not defined in syst-gen.tex but in context.tex. This is because syst-gen.tex is sometimes loaded under Latex.
The expansion of the macro is something like this:
Conditional execution of engine-specific code
There are a few macrosthat delimit code that is executed conditionally. \beginTEX
and \beginETEX
are mutually exclusive, depending on whether or not the format file was compiled under an e-TeX-enabled executable. A typical way of setting up your code to use e-TeX where available is like this:
\beginTEX \def\ifundefined#1% {\expandafter\ifx\csname#1\endcsname\relax} \endTEX \beginETEX \ifcsname \def\ifundefined#1% {\unless\ifcsname#1\endcsname} \endETEX
Code delimited by \beginOMEGA ... \endOMEGA
is only executed if ConTeXt runs under Omega, \beginXETEX ... \endXETEX
only if XeTeX is used, ..., and ignored otherwise.
The optional argument after the \begin...
can be used to give information to the viewer: the example above will print the following string to the terminal:
system (E-TEX) : [line 833] \ifcsname
Guard against double-loading of input files
Because modules can be used in various contexts, we want to be able to prevent macro files from being loaded more than once. This can be done using:
\abortinputifdefined\command
where \command
is a command defined in the module to be loaded only once.
For example, syst-gen.tex implements \writestatus
, and therefore it starts with:
\abortinputifdefined\writestatus
Actually you don't need this macro for modules, since \usemodule
does it's own bookkeeping. It is intended for files that are loaded via the TeX primitive \input
.
Protecting internal macros
We can shield macros from users by using some special characters in their names. Some characters that TeX normally does not consider to be letters (and therefore used) are: @
, !
and ?
. Before and after the definition of protected macros, we have to change the <catcode> of these characters. This is done by \unprotect
and \protect
, for instance:
\unprotect \def\!test{alfa} \protect
The newly defined command \!test
can of course only be called upon when we are in the \unprotect
'ed state, otherwise TeX reads the command \!
, followed by the word test
and probably complains loudly about not being in math mode.
The protection/unprotection commands can be nested (unlike \makeatletter
in LaTeX). This nesting is a convenience, since it allows one to use the protection pair regardless of whether protection is already turned on.
When the nesting becomes deeper than one level, the system reports the current protection level.
It is a good habit to always start your macro files with \unprotect
and end them with \protect
.
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