How to set a key mapping in the modeline?

Question

I have a text file where I keep my personal notes. They're structured in a certain way, so I can operate on those notes with vim commands. I would like to map a keyboard shortcut to a specific set of commands, but that mapping should be available only in this file. So I tried writing a modeline at the end of my file, like this:

# vim: nmap \a A [a]<ESC>

Unfortunately, that didn't work: when opening the file, vim says

Error detected while processing modelines:
line  273:
E518: Unknown option: nmap

How do I set a mapping that only works in this file? Preferably without writing to .vimrc or another file. Also, my file doesn't have a filetype, it's just text, and the file doesn't have any extension.

Answer 1

Moving to answers, as requested.

Modelines are run in a restricted sandbox, there is only a limited number of things they can do, and defining keyboard maps is not among those.

You can however set a filetype in a modeline, then define a map in a corresponding ftplugin file. If you add the <buffer> qualifier the map will be defined only for the buffers of that filetype (cf. :h :map-<buffer>).

Also, the filetype doesn't have to be a "well-known" name, a made-up name would work just as well as a well-known one.