20.2 Defining Commands

The special form interactive turns a Lisp function into a command. The interactive form must be located at top-level in the function body, usually as the first form in the body; this applies to both lambda expressions (see Lambda Expressions) and defun forms (see Defining Functions). This form does nothing during the actual execution of the function; its presence serves as a flag, telling the Emacs command loop that the function can be called interactively. The argument of the interactive form specifies how the arguments for an interactive call should be read.

Alternatively, an interactive form may be specified in a function symbol’s interactive-form property. A non-nil value for this property takes precedence over any interactive form in the function body itself. This feature is seldom used.

Sometimes, a function is only intended to be called interactively, never directly from Lisp. In that case, give the function a non-nil interactive-only property, either directly or via declare (see Declare Form). This causes the byte compiler to warn if the command is called from Lisp. The output of describe-function will include similar information. The value of the property can be: a string, which the byte-compiler will use directly in its warning (it should end with a period, and not start with a capital, e.g., "use (system-name) instead."); t; any other symbol, which should be an alternative function to use in Lisp code.