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4.3.1 Binding constructs for syntactic keywords

Let-syntax’ and ‘letrec-syntax’ are analogous to ‘let’ and ‘letrec’, but they bind syntactic keywords to macro transformers instead of binding variables to locations that contain values. Syntactic keywords may also be bound at top level; see section Syntax definitions.

syntax: let-syntax <bindings> <body>

Syntax: <Bindings> should have the form

((<keyword> <transformer spec>) …,)

Each <keyword> is an identifier, each <transformer spec> is an instance of ‘syntax-rules’, and <body> should be a sequence of one or more expressions. It is an error for a <keyword> to appear more than once in the list of keywords being bound.

Semantics: The <body> is expanded in the syntactic environment obtained by extending the syntactic environment of the ‘let-syntax’ expression with macros whose keywords are the <keyword>s, bound to the specified transformers. Each binding of a <keyword> has <body> as its region.

(let-syntax ((when (syntax-rules ()
                     ((when test stmt1 stmt2 ...)
                      (if test
                          (begin stmt1
                                 stmt2 ...))))))
  (let ((if #t))
    (when if (set! if 'now))
    if))                               ⇒  now

(let ((x 'outer))
  (let-syntax ((m (syntax-rules () ((m) x))))
    (let ((x 'inner))
      (m))))                           ⇒  outer
syntax: letrec-syntax <bindings> <body>

Syntax: Same as for ‘let-syntax’.

Semantics: The <body> is expanded in the syntactic environment obtained by extending the syntactic environment of the ‘letrec-syntax’ expression with macros whose keywords are the <keyword>s, bound to the specified transformers. Each binding of a <keyword> has the <bindings> as well as the <body> within its region, so the transformers can transcribe expressions into uses of the macros introduced by the ‘letrec-syntax’ expression.

(letrec-syntax
  ((my-or (syntax-rules ()
            ((my-or) #f)
            ((my-or e) e)
            ((my-or e1 e2 ...)
             (let ((temp e1))
               (if temp
                   temp
                   (my-or e2 ...)))))))
  (let ((x #f)
        (y 7)
        (temp 8)
        (let odd?)
        (if even?))
    (my-or x
           (let temp)
           (if y)
           y)))                        ⇒  7

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