Книга: Learning GNU Emacs, 3rd Edition

11.3.5 Finding Other Built-in Functions

11.3.5 Finding Other Built-in Functions

Emacs contains hundreds of built-in functions that may be of use to you in writing Lisp code. Yet finding which one to use for a given purpose is not so hard.

The first thing to realize is that you will often need to use functions that are already accessible as keyboard commands. You can use these by finding out what their function names are via the C-h k (for describe-key) command (see Chapter 14). This gives the command's full documentation, as opposed to C-h c (for describe-key-briefly), which gives only the command's name. Be careful: in a few cases, some common keyboard commands require an argument when used as Lisp functions. An example is forward-word; to get the equivalent of typing M-f, you have to use (forward-word 1).

Another powerful tool for getting the right function for the job is the command-apropos(C-h a) help function. Given a regular expression, this help function searches for all commands that match it and display their key bindings (if any) and documentation in a *Help* window. This can be a great help if you are trying to find a command that does a certain "basic" thing. For example, if you want to know about commands that operate on words, type C-h a followed by word, and you will see documentation on about a dozen and a half commands having to do with words.

The limitation with command-apropos is that it gives information only on functions that can be used as keyboard commands. Even more powerful is apropos, which is not accessible via any of the help keys (you must type M-x apropos Enter). Given a regular expression, apropos displays all functions, variables, and other symbols that match it. Be warned, though: apropos can take a long time to run and can generate very long lists if you use it with a general enough concept (such as buffer).

You should be able to use the apropos commands on a small number of well-chosen keywords and find the function(s) you need. Because, if a function seems general and basic enough, the chances are excellent that Emacs has it built-in.

After you find the function you are interested in, you may find that the documentation that apropos prints does not give you enough information about what the function does, its arguments, how to use it, or whatever. The best thing to do at this point is to search Emacs's Lisp source code for examples of the function's use. "A Treasure Trove of Examples" earlier in this chapter provides ways of finding out the names of directories Emacs loads libraries from and an easy way of looking at a library once you know its name. To search the contents of the library files you'll need to use grep or some other search facility to find examples, then edit the files found to look at the surrounding context. If you're ambitious you could put together the examples and concepts we've discussed so far to write an extension of the find-library-file command that searches the contents of the library files in each directory on the load path! Although most of Emacs's built-in Lisp code is not profusely documented, the examples of function use that it provides should be helpful—and may even give you ideas for your own functions.

By now, you should have a framework of Emacs Lisp that should be sufficient for writing many useful Emacs commands. We have covered examples of various kinds of functions, both Lisp primitives and built-in Emacs functions. You should be able to extrapolate many others from the ones given in this chapter along with help techniques such as those just provided. In other words, you are well on your way to becoming a fluent Emacs Lisp programmer. To test yourself, start with the code for count-words-buffer and try writing the following functions:

count-lines-buffer

Print the number of lines in the buffer.

count-words-region

Print the number of words in a region.

what-line

Print the number of the line point is currently on.

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