25.2.1 Using interactive

This section describes how to write the interactive form that makes a Lisp function an interactively-callable command.

There are three possibilities for the argument arg-descriptor:

• It may be omitted or nil; then the command is called with no arguments. This leads quickly to an error if the command requires one or more arguments.
• It may be a Lisp expression that is not a string; then it should be a form that is evaluated to get a list of arguments to pass to the command. If this expression reads keyboard input (this includes using the minibuffer), keep in mind that the integer value of point or the mark before reading input may be incorrect after reading input. This is because the current buffer may be receiving subprocess output; if subprocess output arrives while the command is waiting for input, it could relocate point and the mark.

  Here's an example of what not to do:

            (interactive
             (list (region-beginning) (region-end)
                   (read-string "Foo: " nil 'my-history)))
  

  Here's how to avoid the problem, by examining point and the mark only after reading the keyboard input:

            (interactive
             (let ((string (read-string "Foo: " nil 'my-history)))
               (list (region-beginning) (region-end) string)))
  

• It may be a string; then its contents should consist of a code character followed by a prompt (which some code characters use and some ignore). The prompt ends either with the end of the string or with a newline. Here is a simple example:

            (interactive "bFrobnicate buffer: ")
  

  The code letter ‘b’ says to read the name of an existing buffer, with completion. The buffer name is the sole argument passed to the command. The rest of the string is a prompt.

  If there is a newline character in the string, it terminates the prompt. If the string does not end there, then the rest of the string should contain another code character and prompt, specifying another argument. You can specify any number of arguments in this way.

  The prompt string can use ‘%’ to include previous argument values (starting with the first argument) in the prompt. This is done using format (see Formatting Strings). For example, here is how you could read the name of an existing buffer followed by a new name to give to that buffer:

            (interactive "bBuffer to rename: \nsRename buffer %s to: ")
  

  If the first character in the string is ‘*’, then an error is signaled if the buffer is read-only.

  If the first character in the string is ‘@’, and if the key sequence used to invoke the command includes any mouse events, then the window associated with the first of those events is selected before the command is run.

  If the first character in the string is ‘_’, then this command will not cause the region to be deactivated when it completes; that is, zmacs-region-stays will be set to t when the command exits successfully.

  You can use ‘*’, ‘@’, and ‘_’ together; the order does not matter. Actual reading of arguments is controlled by the rest of the prompt string (starting with the first character that is not ‘*’, ‘@’, or ‘_’).