Derived major mode and shared filename extension

[Johan Vromans]

Hi,

I have a major mode 'joplin-mode' that is derived from 'markdown-mode'. A
Joplin data file is basically a Markdown document with some twists, and
shares the extension '.md'. Whether a document is joplin can be determined
from its content.

What is the proper way to devise this?

One alternative is to add joplin-mode to auto-mode-alist as the handler for
'.md'. When joplin-mode detects that it is not a joplin file it changes the
mode-name back to "Markdown" and the buffer will be handled by the parent
markdown-mode.

Another alternative is to add markdown-mode to auto-mode-alist and add a
hook to markdown-mode-hooks to inspect the contents, and if it is joplin
data then invoke joplin-mode. (Which will invoke markdown-mode so I must
prevent unwanted recursion. That can be solved.)

Is either approach preferred? Or is there an even better way to do it?

Thanks,
Johan