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(librep.info)Defining Macros

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Defining Macros
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   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.

 - Macro: 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 (Note: 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 the definition of the
`when' macro shown in the previous section.

     (defmacro when (condition #!rest body)
       "Evaluates CONDITION, if it's true evaluates the BODY
     forms."
       (list 'cond (list* condition body)))

When a form of the type `(when C B ...)' is evaluated the macro
definition of `when' expands to the form `(cond (C (progn B ...)))'
which is then evaluated to perform the `when'-construct.

   When you define a macro ensure that the forms which produce the
expansion have no side effects; otherwise undefined effects will occur
when programs using the macro are compiled.