Книга: Learning GNU Emacs, 3rd Edition

6.2 Tips for Creating Good Macros

6.2 Tips for Creating Good Macros

It's easy to learn how to record and reuse your keystrokes. However, when you're starting out, you make a few mistakes: you create a macro, use it, and then find out that it doesn't do exactly what you thought. With a little care, it's easy to make your macros more useful and less vulnerable to mistakes.

Good macros work in all situations. Therefore, within a macro, you should use commands that are absolute rather than relative. For example, if you write a macro that puts a formatting string around the word the cursor is on, you want the macro to work no matter how long the word is. Therefore, you would use an absolute command such as M-f (for forward-word) rather than a few C-fs to move forward one character at a time. Similarly, commands such as C-e and C-a are good for finding the beginning or end of a line rather than moving the cursor forward or backward.

Often, macros start with a search command that brings you to the place in the file you want the macro to start. It's a good idea to type the search argument (as in C-s searchstring) rather than using the command to repeat the last search (C-s C-s). You may have changed the search string between the time you define the macro and the time you execute it, and C-s C-s remembers only what the last search string was.

It is often a good idea to add extra commands (typically C-a and C-e) that aren't strictly necessary, just to make sure that you're positioned correctly on the line. The fewer assumptions that a macro makes, the better it works. So, if a sequence of commands works correctly only if you start at the end of the line, start the macro with C-e, even if you already "know" that you want to give the command only when you're at the end of the line.

Finally, while we're reciting rules and cautions, here's one more: keep in mind that you probably want to execute macros repeatedly. With a little foresight, you'll be able to create macros that can be executed in long chains without problems.

In general, good macros have three parts:

• They find the place you want the macro to start working (often using search).

• They do the work that needs to be done on the text.

• They prepare themselves to repeat.

How can a macro prepare itself to repeat? For example, assume that you're writing a macro to delete the third column of a table. After deleting the column, the macro should position itself at the beginning of the next line (or wherever it needs to be) so you don't have to reposition the cursor before reusing it.

Here's a slightly more complex example. If you start a macro with a search, you have to make sure that the end of the macro moves the cursor past the last spot you searched for. If you don't, the macro will keep finding the same place in the file and never go on to the next occurrence of what you're searching for. As a general rule, if your macro operates on a line of text, it should end by moving to the beginning of the next line. Remember that your goal is to create a sequence of keystrokes that can be executed many times in a row, with no interruption.

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