Using Interactive

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21.2.1 Using `interactive`

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

— Special Form: interactive arg-descriptor

This special form declares that a function is a command, and that it may therefore be called interactively (via M-x or by entering a key sequence bound to it). The argument arg-descriptor declares how to compute the arguments to the command when the command is called interactively.
A command may be called from Lisp programs like any other function, but then the caller supplies the arguments and arg-descriptor has no effect.
The interactive form must be located at top-level in the function body, or in the function symbol's interactive-form property (see Symbol Properties). It has its effect because the command loop looks for it before calling the function (see Interactive Call). Once the function is called, all its body forms are executed; at this time, if the interactive form occurs within the body, the form simply returns nil without even evaluating its argument.
By convention, you should put the interactive form in the function body, as the first top-level form. If there is an interactive form in both the interactive-form symbol property and the function body, the former takes precedence. The interactive-form symbol property can be used to add an interactive form to an existing function, or change how its arguments are processed interactively, without redefining the function.

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 string; its contents are a sequence of elements separated by newlines, one for each argument¹. Each element consists of a code character (see Interactive Codes) optionally followed by a prompt (which some code characters use and some ignore). Here is an example:
```
          (interactive "P\nbFrobnicate buffer: ")
```
The code letter ‘P’ sets the command's first argument to the raw command prefix (see Prefix Command Arguments). ‘bFrobnicate buffer:’ prompts the user with ‘Frobnicate buffer: ’ to enter the name of an existing buffer, which becomes the second and final argument.
The prompt string can use ‘%’ to include previous argument values (starting with the first argument) in the prompt. This is done using format-message (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 ‘*’ appears at the beginning of the string, then an error is signaled if the buffer is read-only.
If ‘@’ appears at the beginning of the string, 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 ‘^’ appears at the beginning of the string, and if the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Shift-translation is controlled on the user level by shift-select-mode; see Shift Selection.
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 ‘^’).
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. Usually this form will call various functions to read input from the user, most often through the minibuffer (see Minibuffers) or directly from the keyboard (see Reading Input).
Providing point or the mark as an argument value is also common, but if you do this and read input (whether using the minibuffer or not), be sure to get the integer values of point or the mark after reading. 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 after reading the keyboard input:
```
          (interactive
           (let ((string (read-string "Foo: " nil 'my-history)))
             (list (region-beginning) (region-end) string)))
```
Warning: the argument values should not include any data types that can't be printed and then read. Some facilities save command-history in a file to be read in the subsequent sessions; if a command's arguments contain a data type that prints using ‘#<...>’ syntax, those facilities won't work.
There are, however, a few exceptions: it is ok to use a limited set of expressions such as (point), (mark), (region-beginning), and (region-end), because Emacs recognizes them specially and puts the expression (rather than its value) into the command history. To see whether the expression you wrote is one of these exceptions, run the command, then examine (car command-history).

— Function: interactive-form function

This function returns the interactive form of function. If function is an interactively callable function (see Interactive Call), the value is the command's interactive form (interactive spec), which specifies how to compute its arguments. Otherwise, the value is nil. If function is a symbol, its function definition is used.

Footnotes

[1] Some elements actually supply two arguments.