When writing a new major mode, should it add itself to auto-mode-alist?

Question

I am defining a new major mode, foo-mode, which is closely and only associated with .foo files. Should foo-mode.el fiddle with auto-mode-alist to add this association, or is that the user's responsibility?

Answer 1

Note that if your library is intended to be installed via package.el (or similar) and you have included an autoload cookie for foo-mode, and you choose to implement the auto-mode-alist manipulation, then you will want to use an autoload cookie for that as well. Otherwise the user would need to either load your library before visiting a *.foo file, or else write their own auto-mode-alist entry.

I don't think there's a hard and fast rule about this. I suspect that now that ELPA is pretty standard, it's more common for packages to update auto-mode-alist automatically, to streamline the installation process. I'd say just use your own judgement, and be sure to still provide equivalent manual installation instructions in your commentary.

Answer 2

I'd say yes. If the user has installed foo-mode, they have installed it to use with .foo files. When I run emacs -Q, there are 184 items in the list (including .org, .java, .xml, .letter, and more, so it's not kept too small.