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3.11 Review

In the last few chapters we have introduced a macro and a fair number of functions and special forms. Here they are described in brief, along with a few similar functions that have not been mentioned yet.

Evaluate the last symbolic expression before the current location of point. The value is printed in the echo area unless the function is invoked with an argument; in that case, the output is printed in the current buffer. This command is normally bound to C-x C-e.
Define function. This macro has up to five parts: the name, a template for the arguments that will be passed to the function, documentation, an optional interactive declaration, and the body of the definition.

For example, in Emacs the function definition of dired-unmark-all-marks is as follows.

          (defun dired-unmark-all-marks ()
            "Remove all marks from all files in the Dired buffer."
            (dired-unmark-all-files ?\r))

Declare to the interpreter that the function can be used interactively. This special form may be followed by a string with one or more parts that pass the information to the arguments of the function, in sequence. These parts may also tell the interpreter to prompt for information. Parts of the string are separated by newlines, ‘\n’.

Common code characters are:

The name of an existing buffer.
The name of an existing file.
The numeric prefix argument. (Note that this p is lower case.)
Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one.

See Code Characters for ‘interactive, for a complete list of code characters.

Declare that a list of variables is for use within the body of the let and give them an initial value, either nil or a specified value; then evaluate the rest of the expressions in the body of the let and return the value of the last one. Inside the body of the let, the Lisp interpreter does not see the values of the variables of the same names that are bound outside of the let.

For example,

          (let ((foo (buffer-name))
                (bar (buffer-size)))
             "This buffer is %s and has %d characters."
             foo bar))

Record the values of point and the current buffer before evaluating the body of this special form. Restore the value of point and buffer afterward.

For example,

          (message "We are %d characters into this buffer."
                   (- (point)
                        (goto-char (point-min)) (point))))

Evaluate the first argument to the function; if it is true, evaluate the second argument; else evaluate the third argument, if there is one.

The if special form is called a conditional. There are other conditionals in Emacs Lisp, but if is perhaps the most commonly used.

For example,

          (if (= 22 emacs-major-version)
              (message "This is version 22 Emacs")
            (message "This is not version 22 Emacs"))

The < function tests whether its first argument is smaller than its second argument. A corresponding function, >, tests whether the first argument is greater than the second. Likewise, <= tests whether the first argument is less than or equal to the second and >= tests whether the first argument is greater than or equal to the second. In all cases, both arguments must be numbers or markers (markers indicate positions in buffers).
The = function tests whether two arguments, both numbers or markers, are equal.
Test whether two objects are the same. equal uses one meaning of the word “same” and eq uses another: equal returns true if the two objects have a similar structure and contents, such as two copies of the same book. On the other hand, eq, returns true if both arguments are actually the same object.
The string-lessp function tests whether its first argument is smaller than the second argument. A shorter, alternative name for the same function (a defalias) is string<.

The arguments to string-lessp must be strings or symbols; the ordering is lexicographic, so case is significant. The print names of symbols are used instead of the symbols themselves.

An empty string, ‘""’, a string with no characters in it, is smaller than any string of characters.

string-equal provides the corresponding test for equality. Its shorter, alternative name is string=. There are no string test functions that correspond to >, >=, or <=.

Print a message in the echo area. The first argument is a string that can contain ‘%s’, ‘%d’, or ‘%c’ to print the value of arguments that follow the string. The argument used by ‘%s’ must be a string or a symbol; the argument used by ‘%d’ must be a number. The argument used by ‘%c’ must be an ascii code number; it will be printed as the character with that ascii code. (Various other %-sequences have not been mentioned.)
The setq function sets the value of its first argument to the value of the second argument. The first argument is automatically quoted by setq. It does the same for succeeding pairs of arguments. Another function, set, takes only two arguments and evaluates both of them before setting the value returned by its first argument to the value returned by its second argument.
Without an argument, return the name of the buffer, as a string.
Without an argument, return the name of the file the buffer is visiting.
Return the buffer in which Emacs is active; it may not be the buffer that is visible on the screen.
Return the most recently selected buffer (other than the buffer passed to other-buffer as an argument and other than the current buffer).
Select a buffer for Emacs to be active in and display it in the current window so users can look at it. Usually bound to C-x b.
Switch Emacs's attention to a buffer on which programs will run. Don't alter what the window is showing.
Return the number of characters in the current buffer.
Return the value of the current position of the cursor, as an integer counting the number of characters from the beginning of the buffer.
Return the minimum permissible value of point in the current buffer. This is 1, unless narrowing is in effect.
Return the value of the maximum permissible value of point in the current buffer. This is the end of the buffer, unless narrowing is in effect.