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Re: Doing things only in a particular mode

From: Joost Kremers
Subject: Re: Doing things only in a particular mode
Date: 24 Aug 2015 17:18:00 GMT
User-agent: slrn/pre1.0.0-18 (Linux)

Colin Yates wrote:
> (newbie warning).
> So I understand about (add-hook...) but I can't find the hook I
> want. Basically, I have visual-line-mode turned on globally, but I want
> to disable it when I view the headers in mu4e.
> The buffer is called *mu4e-headers* and I can see the major mode is
> mu4e-headers but the following code has no effect:
> (add-hook 'mu4e-headers-hook
>   (lambda ()
>     (visual-line-mode 0)))

Well, the hook is actually called `mu4e-headers-mode-hook`, so if you
use that, it should work.

To check if a variable exists, or find one if you have some idea what it
might be called, you can use `C-h v`, type the name and hit RET. TAB
completion works, so typing e.g., `C-h v mu4e-headers-hook TAB` would
have found the right variable for you.

BTW, the general advice is to not use lambdas in hook variables, just
function names. You might not really care, but if you want to be
pedantically correct about things, you could write:

(defun my-mu4e-headers-function ()  ; use whatever name you see fit
  (visual-line-mode -1))
(add-hook 'my-mu4e-headers-function)

Note that I use `-1` as the argument to `visual-line-mode`. IIRC an
argument of 0 would actually activate the mode.

> I am not sure how 'hooks' are created - I searched through the source
> code for my4e-headers-hook but couldn't find it.

They are created automatically when you create a major or minor mode
with `define-derived-mode` or `define-minor-mode`, so that's why you
couldn't find it by grepping the source.

> Assuming this is the right approach, how can I say 'when the major mode
> is X then do this'. What is the idiomatic Emacs way?

Well, one can argue about the meaning of "idiomatic", but here's how one
could do it:

(when (eq major-mode 'mu4e-headers-mode)
  (do this)
  (and that))

Note, however, that in a mode major hook, there's no need to use this,
because if the mode were anything else, the hook wouldn't be run. It
could be useful in an Elisp program or in a (function called in a) minor
mode hook, though.


Joost Kremers                                   address@hidden
Selbst in die Unterwelt dringt durch Spalten Licht

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