Debra Cameron - Learning GNU Emacs, 3rd Edition

However, if no suffix is listed (or if your compiler supports a different suffix than the ones listed), you can set up Emacs to invoke the mode automatically when you visit your source files. You need to do two things: first, look again at the right-hand column in the package table entry for your language, and you will find the name of the function that invokes the mode (e.g., ada-mode, modula-2-mode). Second, you insert code in your .emacs file that tells Emacs to automatically load the proper package whenever you visit a file with the suffix for the language in question.

You need to write two lines of code for this customization. The first uses the autoloadfunction, which tells Emacs where to look for commands it doesn't already know about. It sets up an association between a function and the package that implements the function so that when the function is invoked for the first time, Emacs loads the package to get the code. In our case, we need to create an association between a function that invokes a language mode and the package that implements the mode. This shows the format of autoload:

(autoload ' function " filename " " description " t)

Note the single quote preceding functionand the double quotes around filenameand description; for more details on this Lisp syntax, see Chapter 11. If you are a PHP programmer, for example, you can grab the latest Emacs PHP mode from http://sourceforge.net/projects/php-mode/ online. You would then put the following line in your .emacs file:

(autoload 'php-mode "php-mode" "PHP editing mode." t)

This tells Emacs to load the PHP package when the function php-modeis invoked for the first time.

The second line of code completes the picture by creating an association between the suffix for source files in your language and the mode-invoking function so that the function is automatically invoked when you visit a file with the proper suffix. This involves the Emacs global variable auto-mode-alist, covered in Chapter 10; it is a list of associations that Emacs uses to put visited files in modes according to their names. To create such an association for PHP mode so that Emacs puts all files with the suffix .php in that mode, add this line to your .emacs file:

(setq auto-mode-alist (cons '("\\.php$" . php-mode) auto-mode-alist))

This Lisp code sets up the following chain of events when you visit a file whose suffix indicates source code in your programming language. Let's say you visit the file pgm.php . Emacs reads the file, then finds an entry corresponding to the .php suffix in the auto-mode-alistand tries to invoke the associated function php-mode. It notices that the function php-modedoesn't exist, but that there is an autoloadassociation between it and the PHP package. It loads that package and, finding the php-modecommand, runs it. After this, your buffer is in PHP mode.

For some interpreted languages like Perl and Python, you will also want to update the interpreter-mode-alist globalvariable:

(setq interpreter-mode-alist

(cons '("python" . python-mode) interpreter-mode-alist))

If your script file begins with the Unix interpreter prefix #!, Emacs checks that line to determine what language you are using. That can be especially helpful when the script file does not have a telltale extension like .py or .pl .

Although language modes differ in exact functionality, they all support the same basic concepts. The most important of these involves knowledge of the syntax of the language in question—its characters, vocabulary, and certain aspects of its grammar. We have already seen that Emacs handles some syntactic aspects of human language. When you edit regular text, Emacs knows about words, sentences, and paragraphs: you can move the cursor and delete text with respect to those units. It also knows about certain kinds of punctuation, such as parentheses: when you type a right parenthesis, it "flashes" the matching left parenthesis by moving the cursor there for a second and then returning. [59]This is a convenient way of ensuring that your parentheses match correctly.

Emacs has knowledge about programming language syntax that is analogous to its knowledge of human language syntax. In general, it keeps track of the following basic syntactic elements:

• Words , which correspond to identifiers and numbers in most programming languages.

• Punctuation , which includes such things as operators (e.g., +, -, <, and >) and statement separators (e.g., semicolons).

• Strings , which are strings of characters to be taken literally and surrounded by delimiters (such as quotation marks).

• Parentheses , which can include such things as square brackets ( [and ]) and curly braces ( {and }) as well as regular parentheses.

• Whitespace , such as spaces and tabs, which are to be ignored.

• Comments , which are strings of characters to be ignored and surrounded by delimiters that depend on the language (e.g., /*and */for C, //and a newline for C++ and Java, or semicolon ( ;) and a newline for Lisp).

Emacs keeps this information internally in the form of syntax tables ; like keymaps (as described in Chapter 10), Emacs has a global syntax table used for all buffers, as well a local table for each buffer, which varies according to the mode the buffer is in. You can view the syntax table for the current buffer by typing C-h s(for describe-syntax). In addition, language modes know about more advanced language-dependent syntactic concepts like statements, statement blocks, functions, subroutines, Lisp syntactic expressions, and so on.

All programming languages have comment syntax, so Emacs provides a few features that deal with comments in general; these are made language-specific in each language mode. The universal comment command for all language modes is M-;(for indent-for-comment). [60]When you type M-;, Emacs moves to a column equal to the value of the variable comment-column; if the text on the line goes past that column, it moves to one space past the last text character. It then inserts a comment delimiter (or a pair of opening and closing delimiters, as in /* and */ for C) and puts the cursor after the opening delimiter.

For example, if you want to add a comment to a statement, put the cursor anywhere on the line containing that statement and type M-;. The result is

result += y; /* _ */

You can then type your comment in between the delimiters. If you were to do the same thing on a longer line of code, say,

q_i = term_arr[i].num_docs / total_docs;

the result would be

q_i = term_arr[i].num_docs / total_docs; /* _ */

You can customize the variable comment-column, of course, by putting the appropriate code in your .emacs file. This is the most useful way if you want to do it permanently. But if you want to reset comment-columntemporarily within the current buffer, you can just move the cursor to where you want the comment column to be and type C-x ;(for set-comment-column). Note that this command affects only the value of comment-columnin the current buffer; its value in other buffers—even other buffers in the same mode—is not changed.