How can I define an Elisp function such that it is available only under a specific mode?
(FWIW, at the moment, the mode I want to restrict a function to is comint.)
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2 Answers
You can't. But you can do either of the following:
When the function is invoked, it can test the current mode and do something appropriate. For example, for some modes, or all modes except some modes, it can be a no-op or it can raise an error.
If it is a command, you can ensure that it is not bound to a key in some modes, or in all modes except some modes.
Well, there is a way to make a function or variable essentially invisible in a given context, and that is to put its symbol in a different obarray from the default obarray. You can think of an obarray as a kind of a namespace.
If you are a relative novice then I wouldn't recommend this, but if this is really what you need then go for it. If you want an example of such use, you can take a look at library synonyms.el (description).
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This is a great answer. I wish I could give it some extra points or something...– kjoCommented Feb 6, 2016 at 16:50
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@kjo Note that while you can test the mode in the function, you should not: a function that makes sense in comint mode surely makes sense in derived modes as well. If the function has a requirement such as needing a process associated with the buffer, test for that (if you need to — in many cases you don't need to test anything, you can just assume things are set up properly and let an exception be thrown if they aren't). Commented Feb 6, 2016 at 19:15
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@Gilles: I think you're advocating to go with EAFP rather than LBYL. I used to agree with this preference, but over the years I've seen EAFP fail in truly spectacular, catastrophic ways, so I've gradually returned to LBYL, to the point that it has become almost reflexive for me. Now I will adopt EAFP only as an exception, if the case for it over LBYL is extremely strong.– kjoCommented Feb 6, 2016 at 20:16
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For anyone else who is reading kjo's comment and doesn't know the acronyms, I'll save you a Google search: EAFP is "Easier to ask for forgiveness than permission" and LBYL is "Look before you leap". Commented Mar 13, 2016 at 23:12
Normally you simply avoid binding the command to any key maps except for the modes you want it to be used in. But that doesn't stop the user from using "m-x" to run the command by name.
The convention on function names for mode-specific commands is mode-whatever, such as dired-do-rename-file. It is unlikely that the user will type that name, unless they really want it.
When defining your mode-only command with defun, you can give a list of applicable modes to the interactive declaration; this will prevent command name completion so that "m-x" won't see it elsewhere. This feature doesn't get used very often. For example, it is not used by dired or org modes, which are full of commands that don't work properly outside of buffers in their respective modes.
Your function can also try to test which mode(s) it is in, and do nothing unless it's the right mode. But the approach of figuring out the mode can be problematic, and you don't want to have to include such boilerplate at the beginning of all your functions. That's not the emacs way.
