3.4 Different Options for interactive

In the example, multiply-by-seven used "p" as the argument to interactive. This argument told Emacs to interpret your typing either C-u followed by a number or META followed by a number as a command to pass that number to the function as its argument. Emacs has more than twenty characters predefined for use with interactive. In almost every case, one of these options will enable you to pass the right information interactively to a function. (See Code Characters for interactive in The GNU Emacs Lisp Reference Manual.)

Consider the function zap-to-char. Its interactive expression is

(interactive "p\ncZap to char: ")

The first part of the argument to interactive is ‘p’, with which you are already familiar. This argument tells Emacs to interpret a prefix, as a number to be passed to the function. You can specify a prefix either by typing C-u followed by a number or by typing META followed by a number. The prefix is the number of specified characters. Thus, if your prefix is three and the specified character is ‘x’, then you will delete all the text up to and including the third next ‘x’. If you do not set a prefix, then you delete all the text up to and including the specified character, but no more.

The ‘c’ tells the function the name of the character to which to delete.

More formally, a function with two or more arguments can have information passed to each argument by adding parts to the string that follows interactive. When you do this, the information is passed to each argument in the same order it is specified in the interactive list. In the string, each part is separated from the next part by a ‘\n’, which is a newline. For example, you can follow ‘p’ with a ‘\n’ and an ‘cZap to char: ’. This causes Emacs to pass the value of the prefix argument (if there is one) and the character.

In this case, the function definition looks like the following, where arg and char are the symbols to which interactive binds the prefix argument and the specified character:

(defun name-of-function (arg char)
  (interactive "p\ncZap to char: ")

(The space after the colon in the prompt makes it look better when you are prompted. See The Definition of copy-to-buffer, for an example.)

When a function does not take arguments, interactive does not require any. Such a function contains the simple expression (interactive). The mark-whole-buffer function is like this.

Alternatively, if the special letter-codes are not right for your application, you can pass your own arguments to interactive as a list.

See The Definition of append-to-buffer, for an example. See Using Interactive in The GNU Emacs Lisp Reference Manual, for a more complete explanation about this technique.