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GNU Emacs Lisp Reference Manual

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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 commands's interactive form.

Special Form: interactive arg-descriptor
This special form declares that the function in which it appears 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 has its effect because the command loop (actually, its subroutine call-interactively) scans through the function definition looking for it, before calling the function. Once the function is called, all its body forms including the interactive form are executed, but at this time interactive simply returns nil without even evaluating its argument.

There are three possibilities for the argument arg-descriptor:

Function: interactive-form function
This function returns the interactive form of function. If function is a command (see section 21.3 Interactive Call), the value is a list of the form (interactive spec), where spec is the descriptor specification used by the command's interactive form to compute the function's arguments (see section 21.2.1 Using interactive). If function is not a command, interactive-form returns nil.

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