I wanted to add to a hook so that
whitespace-mode is enabled whenever I edit makefiles.
M-x describe-mode says that
In addition to any hooks its parent mode might have run, this mode runs the hook ‘makefile-gmake-mode-hook’, as the final or penultimate step during initialization.
I added the following line to
(add-hook 'makefile-gmake-mode 'whitespace-mode)
M-x eval-region on the new line. But
whitespace-mode did not become enabled when editing makefiles! This puzzled me for several frustrating minutes before I realised that I had used the wrong hook name. I should have written
(add-hook 'makefile-gmake-mode-hook 'whitespace-mode)
What I really want in this situation is a type error. Of course, I’m never going to get one when writing Emacs lisp. I would settle for a run time error, but neither was one of those forthcoming. Why on earth not?!
A StackOverflow question and answer hint at the underlying rationale. Emacs supports setting hook variables for packages that have not been loaded yet. It must be possible to add hooks to arbitrary symbols, even incorrectly spelled ones! To me this seems to be a consequence of Emacs’s 1970s roots; we hadn’t yet learned that programming via big blobs of global mutable state should be avoided.