defalias derived mode to parent mode

Question

I wrote a new major mode config-general-mode, which is derived from conf-mode. I like it a lot and want to have it as the default mode for all configuration files. Thus I tried:

(defalias 'conf-mode 'config-general-mode)

But this doesn't work, I get the error message:

Lisp nesting exceeds `max-lisp-eval-depth'

Of course I could (and probably will) just associate *.cfg and *.conf with my mode, but I am curious, if such an alias can be made possible somehow.

Answer 1

The nature of a derived mode is that its parent is called before the body of the child mode is evaluated¹.

Therefore if the child becomes its own parent (or grandparent, etc...), that causes infinite recursion.

You might attempt to hack your way around this, but I would firmly suggest that you just "don't do that".

¹ For more details on how derived modes work, see "Derived modes, and mode hooks" in https://stackoverflow.com/a/19295380

Answer 2

Here's how you can do what you intended, while avoiding the inf-loop:

(defalias 'old-conf-mode (symbol-function 'conf-mode))
(define-derived-mode conf-mode old-conf-mode ...)

Note that this is still pretty risky: e.g. it won't work right if conf-mode is marked for autoloading when the above code is executed, and it can lead to inf-loops if you re-execute that code a second time.

A safer alternative would be to use advice-add to modify conf-mode rather than defining a separate conf-general-mode. But it'd still have some quirks (e.g. running conf-mode-hook too early).