Defining Macros

Macros are defined in the same style as functions, the only difference is the name of the special form used to define them.

A macro object is a list whose car is the symbol macro, its cdr is the function which creates the expansion of the macro when applied to the macro calls unevaluated arguments.

Special Form: defmacro name lambda-list body-forms...

Defines the macro stored in the function cell of the symbol name. lambda-list is the lambda-list specifying the arguments to the macro (see section Lambda Expressions) and body-forms are the forms evaluated when the macro is expanded. The first of body-forms may be a documentation string describing the macro's use.

Here is a simple macro definition, it is a possible definition for the when construct (which might even be useful if when wasn't already defined as a special form...),

(defmacro when (condition &rest body)
  "Evaluates condition, if it's non-nil evaluates the body
forms."
  (list 'if condition (cons 'progn body)))

When a form of the type (when c b ...) is evaluated the macro definition of when expands to the form (if c (progn b ...)) which is then evaluated to perform my when-construct.

When you define a macro ensure that the forms which produce the expansion have no side effects; it would fail spectacularly when you attempt to compile your program!

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