Now when you execute LaTeX it quickly restores it’s internal state from latex.fmt, and processes your file. TeX’s solution to this is to run through latex.ltx, then dump it’s internal internal memory to disk in a format file, conventionally latex.fmt. Parsing and executing this file just to typeset a 10 line document would make LaTeX prohibitively slow, especially on the 1980’s era CPUs for which TeX was designed. The version of latex.ltx included in the iOS version of Texifier, our TeX editor is 8000 lines long. Having clarified that point I should now correct a small lie in the previous, TeX does not actually load the latex.ltx afresh each time it typesets a LaTeX file, it restores it’s internal memory of a precompiled version of latex.ltx, known as the format file. Unfortunately Plain TeX has incorrectly become synonymous with TeX over the years which can lead to a lot of confusion.
From now on I will be careful to make a distinction between TeX, the executable and typesetting language, and Plain TeX, which was the first environment written in TeX, and defined in the plain.tex file. That is to say when you type LaTeX you are running TeX, but before it loads your LaTeX source it first loads the latex.ltx file, which defines all the familiar \section, \documentclass, \begin, \end, \ref, \label, … commands familiar to LaTeX users. LaTeX and Plain TeX are two different customised typesetting environments, with their own commands and conventions, based on TeX. Having just added Plain TeX support to our iOS LaTeX editor, I will take this moment to explain the difference.įirst, it is necessary to understand that TeX is not just a typesetter, it is a typesetter with an embedded scripting language, whose commands configure and control the behaviour of the typesetter. You may have read that LaTeX is “really just TeX with macros”, and it is true, but is a simple statement that doesn’t explain why TeX and LaTeX are separate executables, and why documents that will typeset with TeX won’t typeset with LaTeX and vice versa.
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