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An identifier macro is a macro that is not ``applied'' to
syntactic arguments. Instead, an identifier macro identifier is
directly replaced with its value whenever the identifier is in an
expression position. Identifier macros are defined with
define-id-macro:
(define-id-macro name value-expr)
The value-expr expression can evaluate to any value. When the
identifier name is encountered during compilation, it is
compiled as if the result of value-expr is in the place of
name. Like define-macro, identifier macros defined with
define-id-macro (at the top-level) are bound in the current
namespace, and local identifier macros are defined with
let-id-macro.
For example, the following expression uses x to automatically
unbox the value in b:
(let ([b (box 5)])
(let-id-macro x `(unbox b)
(display x) (newline)
(set-box! b 8)
(display x) (newline)))
Each use of x is replaced by (unbox b), so this expression
prints 5 and 8 to the current output port. The x
identifier is not a variable; (set! x 8) is illegal, since
this expands to (set! (unbox b) 8). The value of the
identifier macro x is the S-expression '(unbox b). Leaving out the quote in defining x's value is illegal:
(let ([b (box 5)])
(let-id-macro x (unbox b)
expr))
because the (unbox b) expression is evaluated at compile time
and is not in the scope of b. (If b is a global variable
bound to a box when the expression is compiled, then the expression
is legal and the global b is used.)
As with let-macro, the let-id-macro form defines a local identifier macro and an internal define-id-macro expression is transformed into a let-id-macro expression.
(id-macro? v) returns #t if v is an identifier macro created with define-id-macro, #f otherwise. Note that id-macro? cannot be applied directly to identifier macro identifiers, but identifier macro values can be obtained indirectly with global-defined-value.
A define-id-macro expression is treated specially by compile-file (see section 15.2.5).