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Programming in Emacs Lisp

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5.3.1 Optional Arguments

Unless told otherwise, Lisp expects that a function with an argument in its function definition will be called with a value for that argument. If that does not happen, you get an error and a message that says `Wrong number of arguments'.

However, optional arguments are a feature of Lisp: a keyword may be used to tell the Lisp interpreter that an argument is optional. The keyword is &optional. (The `&' in front of `optional' is part of the keyword.) In a function definition, if an argument follows the keyword &optional, a value does not need to be passed to that argument when the function is called.

The first line of the function definition of beginning-of-buffer therefore looks like this:

 
(defun beginning-of-buffer (&optional arg)

In outline, the whole function looks like this:

 
(defun beginning-of-buffer (&optional arg)
  "documentation..."
  (interactive "P")
  (push-mark)
  (goto-char
    (if-there-is-an-argument
        figure-out-where-to-go
      else-go-to
      (point-min))))

The function is similar to the simplified-beginning-of-buffer function except that the interactive expression has "P" as an argument and the goto-char function is followed by an if-then-else expression that figures out where to put the cursor if there is an argument.

The "P" in the interactive expression tells Emacs to pass a prefix argument, if there is one, to the function. A prefix argument is made by typing the META key followed by a number, or by typing C-u and then a number (if you don't type a number, C-u defaults to 4).

The true-or-false-test of the if expression is simple: it is simply the argument arg. If arg has a value that is not nil, which will be the case if beginning-of-buffer is called with an argument, then this true-or-false-test will return true and the then-part of the if expression will be evaluated. On the other hand, if beginning-of-buffer is not called with an argument, the value of arg will be nil and the else-part of the if expression will be evaluated. The else-part is simply point-min, and when this is the outcome, the whole goto-char expression is (goto-char (point-min)), which is how we saw the beginning-of-buffer function in its simplified form.


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