System Macros

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This is an evolving copy of the original article ConTeXt System Macros, part 1: general macros by Taco.

Abstract

All large macro packages for TeX have the need for a number of low-level macros to easy the programming effort. This is definately true for the ConTeXt package, where the extensive use of key--value pairs and the multilingual interface introduce extra complications to the already tricky art of TeX programming.

Some of these internal macros are real gems, and nearly all of them can also be used in your own source documents. Although most of ConTeXt is described in the source code, sometimes the explanations are too technical for a casual user, and some of the documentation, especially the examples, is still in dutch.

This series of articles will try to highlight the most usable commands of the internal macro layer, using english examples, and removing most of the technical stuff (like the definitions of the macros themselves, and the optimization history).

Disclaimer: Quite a lot of the explanation text is copied from source code documentation by Hans Hagen. Always assume that the errors are mine and the good jokes are his.

Introduction

This article will mostly deal with the file syst-gen.tex. All except the first couple of macros appear in this file, which is input immediately after the inclusion of a stripped down version of the plain TeX format. Most of the following macros are very basic, most of them are related to programming constructs like flow control (\if statements), data structures (comma separated lists), and definitions.

System Macros/Fundamentals

System_Macros/Mnemonics_and_Aliases

System_Macros/Scratch_Variables

System_Macros/Expansion_Control

System_Macros/Handling_Arguments

System_Macros/Definitions_and_Assignments

System_Macros/Branches_and_Decisions

System_Macros/Action_Processing

System_Macros/Comma_Separated_Lists

System_Macros/Key_Value_Assignments

User interaction

This macro hardly needs explanation. It stops TeX until you input something.

Macros for showing messages. In the multi-lingual modules, we will also introduce a mechanism for message passing. For the moment we stick to the core macros:

\writestring {string}
\writeline
\writestatus {category} {message}

Messages are formatted: the category appears within a fixed number of columns, on it's own, followed by a colon. One can provide the maximum width of the identification string with the macro \statuswidth.

For debugging purposes we can enhance macros with the next alternative. Here debuggerinfo stands for both a macro accepting two arguments and a boolean (in fact a few macro's too).

\debuggerinfo {subcategory} {message}

This message will only be output if \debuggerinfo is true. (it usually isn't). If you need to calculate something to be used inside the message, embrace your calculating code with \ifdebuggerinfo ... \fi.