How is filetype tex and plaintex set in vanilla Vim?

Question

I noticed that when I create a .tex from within Vim itself, in a later session that file is detected as plaintex, while already existing files are detected with the tex filetype (note: both of them have the .tex file extension).

I noticed it, since the syntax highlighting was different between the two.

I then created after\ftplugin\plaintex.vim in which I added

set filetype=tex

which does change the filetype, but it does not apply the relative syntax highlighting. I guess because this is done after the buffer is already loaded and processed.

I then tried with ftdetect/plaintex.vim with

setfiletype tex
set filetype=tex

It didn't work either.

I think the last option would be to work with either filetype.vim or script.vim to set filetype before built-in detection.

How can I make Vim assign the "tex" filetype when creating .tex files?

_{I've been trying to solve this myself, without success.}

Answer 1

The filetype of a .tex file is determined by the content of the file.

In particular, if the first lines contains:

• %&plaintex then it will be considered plaintex,
• %&tex then it will be considered tex

If no %&format is found:

The filetype is by default set to:

• plaintex if g:tex_flavor is set to plain or not defined
• context if g:tex_flavor is set to context
• tex otherwise

But the logic will further explore the first 1000 lines of the file for LaTeX or ConTeX constructions and set the filetype accordingly.

The list of LaTeX constructions in Vim 9.0 seems to be:

• \documentclass
• \usepackage
• \begin{
• \newcommnand
• \renewcommand

The list of ConTeX constructions in Vim 9.0 seems to be:

• \start
• \setup
• \usemodule
• \enablemode
• \setvariables
• \useenconding
• \usesymbol
• \stelle
• \verwende
• \stel
• \gebruik
• \usa
• \imposta
• \regle
• \utilisemodule

To have it set to tex instead of plaintex I see three solutions:

1. Start the file with %&tex
2. Set the g:tex_flavor to tex
3. Make sure the content of the files contains \documentclass (or any LaTeX constructions)

To have it set to plaintex instead of tex I see two solutions:

1. Start the files with %&plaintex
2. Make sure the content of the file does not contain LaTeX constructions or ConTeX constructions.

More information with :help ft-tex-plugin