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Kawa, the Java-based Scheme system

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7.1 Syntax and conditional compilation

Syntax: define-syntax ..
Pattern ...

Syntax: defmacro name lambda-list form ...
Defines an old-style macro a la Common Lisp, and installs (lambda lambda-list form ...) as the expansion function for name. When the translator sees an application of name, the expansion function is called with the rest of the application as the actual arguments. The resulting object must be a Scheme source form that is futher processed (it may be repeatedly macro-expanded).

If you define a macro with defmacro, you (currently) cannot use the macro in the same compilation as the definition. This restriction does not apply to macros defined by define-syntax.

Function: gentemp
Returns a new (interned) symbol each time it is called. The symbol names are implementation-dependent. (This is not directly macro-related, but is often used in conjunction with defmacro to get a fresh unique identifier.)

Syntax: cond-expand cond-expand-clause* [(else command-or-definition*)]
 
cond-expand-clause ::= (feature-requirement command-or-definition*)
feature-requirement ::= feature-identifier
  | (and feature-requirement*)
  | (or feature-requirement*)
  | (not feature-requirement)
feature-identifier ::= a symbol which is the name or alias of a SRFI

The cond-expand form tests for the existence of features at macro-expansion time. It either expands into the body of one of its clauses or signals an error during syntactic processing. cond-expand expands into the body of the first clause whose feature requirement is currently satisfied; the else clause, if present, is selected if none of the previous clauses is selected.

A feature requirement has an obvious interpretation as a logical formula, where the feature-identifier variables have meaning true if the feature corresponding to the feature identifier, as specified in the SRFI registry, is in effect at the location of the cond-expand form, and false otherwise. A feature requirement is satisfied if its formula is true under this interpretation.

Examples:
 
(cond-expand
    ((and srfi-1 srfi-10)
     (write 1))
    ((or srfi-1 srfi-10)
     (write 2))
    (else))

 
(cond-expand
  (command-line
   (define (program-name) (car (argv)))))

The second example assumes that command-line is an alias for some feature which gives access to command line arguments. Note that an error will be signaled at macro-expansion time if this feature is not present.


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  Copyright 2003   by The Free Software Foundation     Updated Jun 2003