24.1 Expansion passing style macros

bigloo syntax: define-expander name proc

This form defines an expander, name, where proc is a procedure of two arguments: a form to macro-expand, and an expander.

bigloo syntax: define-macro (name [args]…) body

This form is itself macro-expanded into a define-expander form.

Macro expanders cannot be exported or imported since there is no way to specify expanders in a module declaration.

Macros defined with define-expander and define-macro are used by both the compiler and the interpreter.

Here is an example of an expander:

(define-expander when 
   (lambda (x e)
      (match-case x
         ((?- ?test . ?exps)
          (e `(if ,test (begin ,@exps)) e))
         (else
           (error "when" "illegal form" x)))))

(when (> a 0) (print a) a)
   → (if (> a 0) (begin (print a) a))

The same example can written with a define-macro form:

(define-macro (when test . exps)
   `(if ,test (begin ,@exps)))

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