try.scm

;; Scheme version of try ... in ... unless ... from the paper
;; Exceptional Syntax by Benton and Kennedy

(define-syntax try
  (syntax-rules (in unless)
    ((try (var val) body ... ((tag handler ...) ...))
     ((catch #t
        (lambda () (let ((v val))
                 (lambda (k) (k v))))
        (lambda (t . args)
          (lambda (k)
            (case t
              ((tag) handler ...) ...
              (else (apply throw t args))))))
      (lambda (var) body ...)))))

(define-syntax try2 ;; suggestion from mark_weaver
  (syntax-rules ()
    ((try (var val) body ... ((tag handler ...) ...))
     (let/ec k
       (let ((var
              (catch #t
                (lambda () val)
                (lambda (t . args)
                  (case t
                    ((tag) (k (begin handler ...))) ...
                    (else (apply throw t args)))))))
         body ...)))))

(try (file (open-file "/tmp/supersekretdata.txt" "r"))
  (read file)
  ((system-error "you got a system error, bro")))
;; $3 = "you got a system error, bro"
(try (file (open-file "/tmp/try.scm" "r"))
  (read file)
  ((system-error "you got a system error, bro")))
;; Some sexp