14.4 Defining Macros

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.

macro defmacro name args [doc] [declare] body…

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 Argument List). 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 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))