What is the best way to run a function taking arguments, eg. one that would normally be run with run-hook-with-args, inside a normal hook, eg. after-save-hook.
As a simple example, I want to add some-function to after-save-hook here, but allow it to have an additional argument.
(defun some-function (&optional arg)
(if arg 'do-this 'otherwise-do-this))
;; how to run `some-function' with argument here?
(add-hook 'after-save-hook 'some-function nil 'local)
So you've got something like this (I've changed the function name to make it less confusing).
(add-hook 'after-save-hook 'my-function nil 'local)
But you're asking how you can arrange that when after-save-hook calls my-function it passes an argument to it.
Firstly, you can't do that directly, for the probably-obvious reason that after-save-hook is a normal hook and therefore gets run in a way that doesn't provide any facility for passing arguments.
That means you have to add to the hook a function which actually does what you want.
You could create a function which does what you want like so:
(add-hook 'after-save-hook (apply-partially 'my-function ARG) nil 'local)
But approaches along these lines are really messy when it comes to inspecting and manipulating the hook later, so I recommend not doing anything like this.
Honestly, the cleanest approach is very simply to define a named function which does what you want, and add that to the hook.
(defun my-function-do-this ()
"Do This"
'do-this)
(add-hook 'after-save-hook 'my-function-do-this nil 'local)
Related
From C-h f add-hook:
HOOK should be a symbol, and FUNCTION may be any valid function. If
HOOK is void, it is first set to nil. If HOOK's value is a single
function, it is changed to a list of functions.
and from the code:
(defun add-hook (hook function &optional append local)
...
(or (boundp hook) (set hook nil))
(or (default-boundp hook) (set-default hook nil))
...
What is this good for? I assume it's somehow useful or else it wouldn't be there... I just can't think of a good use for it...
It allows you to set hook variables before the packages which define them have been loaded.
following line removes all training white space while saving.
(add-hook 'write-file-hooks 'delete-trailing-whitespace)
but I want to hook this feature only when i'm in programming mode, so i did
(defun nuke_traling ()
(add-hook 'write-file-hooks 'delete-trailing-whitespace)
)
(add-hook 'prog-mode-hook 'nuke_traling)
which doesn't is not stopping which are not in programming mode.
Making the hook variable buffer-local has been mentioned. Don't do that. Or rather, don't do it using make-local-variable.
The normal hook mechanisms have buffer-local support built in -- that's the purpose of the LOCAL argument to add-hook. When the hook is run, it runs both the global and the buffer-local values.
So taking the example code in the question, you could change it to use:
(add-hook 'write-file-hooks 'delete-trailing-whitespace nil t)
And then delete-trailing-whitespace would be called whenever write-file-hooks was run, but only in the buffers in which prog-mode-hook had run.
However there are better ways to achieve this.
I agree with Drew that you are better to test whether your mode is derived from prog-mode, and with juanleon that before-save-hook is a better hook to use. So you might do something like:
(add-hook 'before-save-hook 'my-prog-nuke-trailing-whitespace)
(defun my-prog-nuke-trailing-whitespace ()
(when (derived-mode-p 'prog-mode)
(delete-trailing-whitespace)))
But what I actually recommend is using either ws-trim or ws-butler to take care of this in a smarter way.
Blindly removing all trailing whitespace from a file is a great way to wind up committing loads of unrelated lines to a version-control repository. Both of the libraries mentioned will ensure that your own commits are free of trailing whitespace, without also introducing unwanted modifications elsewhere in the file.
write-file-hooks is obsolete since Emacs-22, replaced by write-file-functions. But this hook is a bit delicate to use (because it can also be used to perform the write), so I recommend you use before-save-hook instead. And to make it apply only to the current buffer, just pass a non-nil value for the local argument of add-hook, as in:
(defun nuke_traling ()
(add-hook 'before-save-hook #'delete-trailing-whitespace nil t))
(add-hook 'prog-mode-hook #'nuke_traling)
Yes, because as soon as you enter a prog-mode mode, you add the function to write-file-hooks, where it remains. And that hook applies to writing any file, regardless of the mode of its buffer.
Instead of putting that simple function on the hook, you can add a function that tests the mode and only does the whitespace deletion when it is a mode where you want to do that.
Or else you would need to make write-file-hooks buffer-local (which I doubt you would want want to do --- the hook is used more generally).
Bad way:
(add-to-list 'write-file-functions 'delete-trailing-whitespace)
Better way is using ws-butler:
(straight-use-package 'ws-butler)
(add-hook 'prog-mode-hook #'ws-butler-mode)
ws-butler-mode remove spaces only on changed lines.
You would need to make the variable buffer local:
(defun nuke_traling ()
(make-variable-buffer-local 'write-file-hooks)
(add-hook 'write-file-hooks 'delete-trailing-whitespace))
But I would recommend using before-save-hook instead:
(defun nuke_traling ()
(add-to-list 'before-save-hook 'delete-trailing-whitespace))
write-file-hooks may be risky if used as a file-local variable, and documentation recomends using before-save-hook instead for thing like you want to do.
In emacs 21 or later you can add this hook to a perticular mode like this:
(add-hook 'prog-mode-hook
(lambda () (add-to-list 'write-file-functions 'delete-trailing-whitespace)))
I want to run a function according to the programming language when I open a source code file using the following. I have to pass the language-specific str to the foo function. How can do it in the add-hook statement?
(defun foo (str)
(blahblah...))
(add-hook 'prog-mode-hook 'foo)
Use the built-in apply-partially:
(add-hook 'prog-mode-hook (apply-partially #'foo "spam with eggs"))
This is described in the manual (first hit on Google for "emacs add-hook"):
(add-hook 'prog-mode-hook (lambda () (foo "foobarbaz")))
The bad news is that mode hooks usually don't have any arguments. The good news is that here's what (emacs) Hooks section of manual has to say about your situation:
Most major modes run one or more "mode hooks" as the last step of initialization.
So, I didn't test it myself, but I'm pretty sure you can write a generic hook that will inspect major-mode variable and do the mode-specific actions instead.
I'm trying to augment the etags-select functions so it will fall-back to a normal find-tag if find-tag at point failed. The code I've tried is:
(defun my-etags-find-tag ()
"Find at point or fall back"
(interactive)
(unless (etags-select-find-tag-at-point)
(etags-select-find-tag)))
(global-set-key (kbd "C-f") 'my-etags-find-tag)
However this fails when point is not at a valid tag. Instead I get a error thrown by etags-select-find-tag-at-point:
etags-select-find-tag-at-point: Wrong type argument: char-or-string-p, nil
In this case I just have to repeat the test done by etags-select-find-tag-at-point:
(defun my-etags-find-tag ()
"Find at point or fall back"
(interactive)
(if (find-tag-default)
(etags-select-find-tag-at-point)
(etags-select-find-tag)))
But it does seem a little redundant. Is it possible to trap exceptions and do alternate processing in elisp?
Try ignore-errors; eg,
(unless (ignore-errors (etags-select-find-tag-at-point))
(etags-select-find-tag))
Normally, (ignore-errors body) returns whatever body returns; when there's error, it returns nil.
Also look at condition-case for more general condition handling.
If you have Elisp info manual installed, you can get more details from
C-hSignore-errors.
Edit:
I failed to consider the possibility that the function may return nil on success; so we probably need
(unless (ignore-errors (or (etags-select-find-tag-at-point) t))
(etags-select-find-tag))
Without modifying the original source code of etags-select.el, I see it the more reasonable option, even when it calls twice to find-tag-default. You can cheat the dynamic environment within the call to avoid the repetition of the call by memoizing it with something like:
(defun my-etags-find-tag ()
"Find at point or fall back"
(interactive)
(let ((ftd (find-tag-default)))
(flet ((find-tag-default () ftd))
(if (find-tag-default)
(etags-select-find-tag-at-point)
(etags-select-find-tag)))))
EDIT: OK, as per your request, an explanation of the code. First, note that this code achieves both questions:
It does not fail
It is more efficient than the code you show (the one you say it is redundant).
Why is it more efficient? The problem with your redundant code is that you call find-tag-default to see if it is nil, and, if it is, you call etags-select-find-tag-at-point. This function calls again to find-tag-default to obtain a default value. What my code does is to cache the value of find-tag-default by redefining the function by being just the value you calculated. The flet does that, so when etags-select-find-tag-at-point calls find-tag-default, the calculated value is returned without any further processing.
https://stackoverflow.com/a/663636/391104
(defun my-c++-mode-hook ()
(setq c-basic-offset 4)
(c-set-offset 'substatement-open 0))
(add-hook 'c++-mode-hook 'my-c++-mode-hook)
Based on my investigation, I just need to add the above code into my .emacs and then it works magically.
Q1> What does defun my-c++-mode-hook () mean? a function definition in lisp?
Q2> What is the usage of following line? where should I trigger it or it is run automatically by emacs
(add-hook 'c++-mode-hook 'my-c++-mode-hook)
Thank you
Q1: Yes, this is a function definition (hence defun). The second symbol is the name, which has the suffix '-hook' to indicate to humans that it is intended to be used as a hook. It could be given (almost) any arbitrary name without changing its behaviour. The empty () indicates the function takes no arguments. Everything else is the body of the function.
Q2: Basically, this adds a pointer to the previous function to the list of functions that are called when ever c++-mode is started. Whenever you start a mode, the Emacs looks for the mode hook, running all the functions in it. Both the function definition and the add-hook line need to go in your .emacs, and they will be run automatically when you start emacs.
To wrap your head around elisp, the introduction is highly recommended. It ships with emacs, and can be accessed from the info system: C-h i, then look for Elisp Introduction.