catch/throw in terms of call/cc

gistfile1.scm

(define throw (lambda (tag val)
                (error "Uncaught throw " tag)))

(define (catch* needle thunk)
  (call/cc (lambda (k)
             (define prev throw)
             (fluid-let ((throw (lambda (try val)
                                  (if (eq try needle)
                                      (k val)
                                      (prev try val)))))
               (thunk)))))

(defmacro catch (tag . body)
  `(catch* ,tag (lambda () ,@body)))