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 23.5 Lisp Style Macro Definitions
 =================================
 
 Macros are objects which cause the expression that they appear in to be
 transformed in some way _before_ being evaluated.  In expressions that
 are intended for macro transformation, the identifier that names the
 relevant macro must appear as the first element, like this:
 
      (MACRO-NAME MACRO-ARGS ...)
 
    In Lisp-like languages, the traditional way to define macros is very
 similar to procedure definitions.  The key differences are that the
 macro definition body should return a list that describes the
 transformed expression, and that the definition is marked as a macro
 definition (rather than a procedure definition) by the use of a
 different definition keyword: in Lisp, `defmacro' rather than `defun',
 and in Scheme, `define-macro' rather than `define'.
 
    Guile supports this style of macro definition using both `defmacro'
 and `define-macro'.  The only difference between them is how the macro
 name and arguments are grouped together in the definition:
 
      (defmacro NAME (ARGS ...) BODY ...)
 
 is the same as
 
      (define-macro (NAME ARGS ...) BODY ...)
 
 The difference is analogous to the corresponding difference between
 Lisp's `defun' and Scheme's `define'.
 
    `false-if-exception', from the `boot-9.scm' file in the Guile
 distribution, is a good example of macro definition using `defmacro':
 
      (defmacro false-if-exception (expr)
        `(catch #t
                (lambda () ,expr)
                (lambda args #f)))
 
 The effect of this definition is that expressions beginning with the
 identifier `false-if-exception' are automatically transformed into a
 `catch' expression following the macro definition specification.  For
 example:
 
      (false-if-exception (open-input-file "may-not-exist"))
      ==
      (catch #t
             (lambda () (open-input-file "may-not-exist"))
             (lambda args #f))
 

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