Sams Teach Yourself Emacs in 24 Hours
Hour 22: Learning Lisp Basics
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Sams Teach Yourself Emacs in 24 Hours |
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Hour 22: Learning Lisp Basics |
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A variable is a container that can hold a value for later retrieval. You have already seen numerous examples of variables: load-path, next-line-add-newlines, and abbrev-file-name to name a few.
Some of the variables can be made buffer-local. That is, their values are different from buffer to buffer. This has the advantage that different major modes can configure Emacs in specific ways. A good example of this are the variables comment-start and comment-end. These variables are used by the filling commands (see Hour 12, "Visible Editing Utilities," for a description of these commands).
When a major mode is started in a given buffer, it makes these two variables buffer-local and sets them to their specific values for the given major mode. Making the variables buffer-local ensures that the variable is configured only for the given buffer and that the variable is not changed in any of the other buffers, which might be in a different major mode.
When a variable is made buffer-local, changes made to it in the given buffer only have an effect in that buffer, not in any of the other buffers. Still a global variable might exist that all the other buffers share (including new ones).
Some variables can be made buffer-local in such a way that new buffers created get a local copy of it. The copy they get is initialized from a default value. An example of such a variable is fill-column. Depending on your intention and whether the variable is buffer-local or not you can use either setq or setq-default. The possible combinations is shown in Table 22.1.
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Intention |
Set Global Value |
Set Local Value |
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The variable is not buffer- local |
This is the most common situation. Here you can simply use setq. You can, however, also use the function setq-default. Thus, it is possible to use this function all the time, when you want to set the global value for a variable. |
In this situation, the variable is not a buffer-local value. To make a buffer-local variable, you must use the function make-local-variable. Given that it is the variable next-line-add-newlines, which you want to make buffer-local and set to nil, use this code: (make-local-variable 'next-line-add newlines) (setq next-line-add newlines nil) |
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The variable is buffer- local |
To set the global value for a buffer-local variable, you must use the function setq-default. |
To set a local value for a buffer-local buffer simply use the function setq. To set a variable local for a given buffer, you must either press M-:.nd execute the commands described previously, or insert the command in a hook (hooks are described in Hour 24, "Installing Emacs Add-Ons"). In short, hooks are commands executed when a major mode, for example, starts. |
If you, for example, want to set the global value for the variable fill-column, you must use setq-default, because this variable is buffer-local.
Some configurations might be local to a given file rather than a given major mode. Examples of these kinds of configurations are
Files used to save abbreviations in. You can have a file in which you have some specific abbreviations that you want defined only for that file. These can be obtained by setting the variable abbrev-file-name local for the given file.
The language used to spell-check a given file. This is very useful if your default dictionary is English, but you now and then need to spell-check a file in another language--for example, Danish.
If you need special keybindings for a given file. You might need to insert some text over and over again in the given file. Then you can record a number of macros and save them to a file and finally bind the macros to easy-to-reach keys (maybe overriding the default keybinding, which you would otherwise use often).
This can be obtained by inserting a few lines at the bottom of the given file. These look like
01 /* Local Variables: */ 02 /* variable1: value1 */ 03 /* variable2: value2 */ 04 /* End: */
In line 1, Local Variables: and End: must be there as is, to ensure that Emacs recognizes these lines as instructions to it. Also in line 1, notice the slashes and asterisks. All the lines must start and end with the same text as this line. These are the comment characters for a C file. variable1 in line 2 is the name of the variable. And the value for this variable is value1, also shown in line 2.
The following is an example where the abbreviation file and the fill-column are set: in a HTML file (enclose a comment in <!-- and -->).
<!-- Local Variables: --> <!-- fill-column: 100 --> <!-- abbrev-file-name: "~/WWW/.abbrevs" --> <!-- End: -->
In the previous examples, loading a file was also mentioned as one of the things you could do with this feature. In general, any kind of Lisp code can be executed. Simply insert a line starting with eval: and include the Lisp expression after that, as can be seen in the following example:
<!-- Local Variables: --> <!-- fill-column: 100 --> <!-- eval: (load-file "~/Emacs/Macros/chapter-macros.el") --> <!-- End: -->
The function load-file reads in a Lisp file and evaluates it.
When Emacs loads a file with local variables, it asks whether you really want this region evaluated. This is to avoid reading in someone else's file, where the file contains code in the eval section that does something you do not like (such as erasing your hard disk). You can ask Emacs to skip this security check if you never edit other people's files. This is done by inserting the following line into your .emacs file:
(setq enable-local-eval t)
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Sams Teach Yourself Emacs in 24 Hours |
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Hour 22: Learning Lisp Basics |
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