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12.14 The declare Form

declare is a special macro which can be used to add meta properties to a function or macro: for example, marking it as obsolete, or giving its forms a special <TAB> indentation convention in Emacs Lisp mode.

— Macro: declare specs...

This macro ignores its arguments and evaluates to nil; it has no run-time effect. However, when a declare form occurs in the declare argument of a defun or defsubst function definition (see Defining Functions) or a defmacro macro definition (see Defining Macros), it appends the properties specified by specs to the function or macro. This work is specially performed by defun, defsubst, and defmacro.

Each element in specs should have the form (property args...), which should not be quoted. These have the following effects:

(advertised-calling-convention signature when)
This acts like a call to set-advertised-calling-convention (see Obsolete Functions); signature specifies the correct argument list for calling the function or macro, and when should be a string indicating when the old argument list was first made obsolete.
(debug edebug-form-spec)
This is valid for macros only. When stepping through the macro with Edebug, use edebug-form-spec. See Instrumenting Macro Calls.
(doc-string n)
This is used when defining a function or macro which itself will be used to define entities like functions, macros, or variables. It indicates that the nth argument, if any, should be considered as a documentation string.
(indent indent-spec)
Indent calls to this function or macro according to indent-spec. This is typically used for macros, though it works for functions too. See Indenting Macros.
(interactive-only value)
Set the function's interactive-only property to value. See The interactive-only property.
(obsolete current-name when)
Mark the function or macro as obsolete, similar to a call to make-obsolete (see Obsolete Functions). current-name should be a symbol (in which case the warning message says to use that instead), a string (specifying the warning message), or nil (in which case the warning message gives no extra details). when should be a string indicating when the function or macro was first made obsolete.
(compiler-macro expander)
This can only be used for functions, and tells the compiler to use expander as an optimization function. When encountering a call to the function, of the form (function args...), the macro expander will call expander with that form as well as with args..., and expander can either return a new expression to use instead of the function call, or it can return just the form unchanged, to indicate that the function call should be left alone. expander can be a symbol, or it can be a form (lambda (arg) body) in which case arg will hold the original function call expression, and the (unevaluated) arguments to the function can be accessed using the function's formal arguments.
(gv-expander expander)
Declare expander to be the function to handle calls to the macro (or function) as a generalized variable, similarly to gv-define-expander. expander can be a symbol or it can be of the form (lambda (arg) body) in which case that function will additionally have access to the macro (or function)'s arguments.
(gv-setter setter)
Declare setter to be the function to handle calls to the macro (or function) as a generalized variable. setter can be a symbol in which case it will be passed to gv-define-simple-setter, or it can be of the form (lambda (arg) body) in which case that function will additionally have access to the macro (or function)'s arguments and it will passed to gv-define-setter.