bound.sls

#!r6rs

(library (bound helper)
  (export make-definition)
  (import (rnrs))

  (define-syntax make-definition
    (lambda(stx2)
      (syntax-case stx2 ()
        ((_ if-bound bound?)
         #'(begin
             (define-syntax if-bound
               (lambda(stx)
                 (syntax-case stx ()
                   ((_ id consequent)
                    #'(if-bound id consequent #f))
                   ((_ id consequent alternate)
                    (not
                     (free-identifier=?
                      (datum->syntax #'if-bound (syntax->datum #'id))
                      #'id))
                    #'consequent)
                   ((_ id consequent alternate)
                    #'alternate))))
             (define-syntax bound?
               (syntax-rules ()
                 ((_ id)
                  (if-bound id #t #f)))))))))
  )

(library (bound)
  (export bound? if-bound)
  (import (bound helper))

  (make-definition if-bound bound?)
  )