The case for scheme macros

scheme_macros

;3-state condition defined as a function
(define (3-state
            value
            positive-body
            zero-body
            negative-body)
    (cond
      ((zero? value) zero-body)
      ((positive? value) positive-body)
      (else negative-body)))

;doesn't work because of eager eval
(3-state 100 (display "P") (display "Z") (display "N")) 

; => NZP


;3-state condition defined as a macro
(define-syntax
    3-state
    (syntax-rules ()
      ((3-state value positive-body zero-body negative-body)
       (cond
         ((zero? value) zero-body)
         ((positive? value) positive-body)
         (else negative-body)))))
         
;does work correctly when pattern is matched and dispatched
(3-state 100 (display "P") (display "Z") (display "N"))

; => P