bound.sls

#!r6rs
;; petite chez scheme と ypsilon で挙動が異なる
(library (bound)
  (export if-bound bound?)
  (import (rnrs))

  (define-syntax if-bound
    (lambda(stx)
      (syntax-case stx ()
        ((_ id consequent)
         #'(if-bound id consequent (begin)))
        ((_ id consequent alternate)
         (not
          (free-identifier=?
           (datum->syntax #'k (syntax->datum #'id))
           #'id))
         #'consequent)
        ((_ id consequent alternate)
         #'alternate))))
  (define-syntax bound?
    (syntax-rules ()
      ((_ id)
       (if-bound id #t #f))))
  )

test.scm

#!r6rs
(import (rnrs) (bound))

(display (bound? sin))
(display (bound? lambda))
(display (bound? syntax))
(display (bound? hoge))

(newline)

;; この定義の有無で以降の場合分け処理が分岐する
;; (define hige "This is hige.\n")

(if-bound hige
          (display "hige was bound.\n")
          (begin
            (display "hige was not bound.\n")
            (define hige "This is alternate hige.\n")))

(display hige)