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R4RS Appendix (define-syntax <keyword> <transformer>) ==> unspecific
S9fES (define-syntax <symbol> <procedure>) ==> unspecific
S9fES (define-syntax (<sym1> <sym2> ...) <body>) ==> unspecific
Syntax: The <keyword> is an identifier, and <transformer> should be
an instance of SYNTAX-RULES.
Semantics: The top-level syntactic environment is extended by binding
the <keyword> to the specified transformer.
(define-syntax let*
(syntax-rules ()
((let* () body1 body2 ...)
(let () body1 body2 ...))
((let* ((name1 val1) (name2 val2) ...)
body1 body2 ...)
(let ((name1 val1))
(let* ((name2 val2) ...)
body1 body2 ...)))))
In addition to the above the DEFINE-SYNTAX implementation of S9fES
allows to bind an ordinary procedure to a syntactic keyword. Its forms
are similar to those of DEFINE. The first form binds a <symbol> to a
<procedure>, which is specified as its second argument. The other form
binds <sym1> to the form
(lambda (<sym2> ...) <body>).
The procedures bound by DEFINE-SYNTAX may be variadic.
Procedures bound by DEFINE-SYNTAX are commonly called "macros". They
receive their arguments in textual (unevaluated) form and they are
called at macro expansion time, i.e. after reading a program but
before evaluating it. When an expression containing a macro application
is being expanded, the macro is applied to its arguments and the
application of the macro is replaced by the value returned by the
macro.
(define-syntax when
(lambda (pred . conseq)
`(if ,pred (begin ,@conseq))))
(when (= 1 1)
(display "true")
#t) ==> #t
Macros may not recurse directly, but they may implement recursion
internally using LETREC or by rewriting their own applications. The
following macro, for example, does not work, because D is undefined
in the body of D:
(define-syntax (d x) (and (pair? x) (d (cdr x)))) ; WRONG!
The following version does work, though, because the macro gets
rewritten to another application of itself, which is subsequently
being expanded:
(define-syntax (d x) (and (pair? x) `(d ,(cdr x)))) ; OK
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