A Lisp macro object is a list whose CAR is macro, and
whose CDR is a function. Expansion of the macro works
by applying the function (with apply) to the list of
unevaluated arguments from the macro call.
It is possible to use an anonymous Lisp macro just like an anonymous
function, but this is never done, because it does not make sense to
pass an anonymous macro to functionals such as mapcar. In
practice, all Lisp macros have names, and they are almost always
defined with the defmacro macro.
defmacro defines the symbol name (which should not be
quoted) as a macro that looks like this:
(macro lambda args . body)
(Note that the CDR of this list is a lambda expression.) This
macro object is stored in the function cell of name. The
meaning of args is the same as in a function, and the keywords
&rest and &optional may be used (see Features of Argument Lists).
Neither name nor args should be quoted. The return value
of defmacro is undefined.
doc, if present, should be a string specifying the macro’s
documentation string. declare, if present, should be a
declare form specifying metadata for the macro (see The declare Form). Note that macros cannot have interactive declarations, since
they cannot be called interactively.
Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ‘`’ syntax (see Backquote). For example:
(defmacro t-becomes-nil (variable)
`(if (eq ,variable t)
(setq ,variable nil)))
(t-becomes-nil foo)
≡ (if (eq foo t) (setq foo nil))