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@thypon thypon/exceptional.rkt
Last active Dec 17, 2015

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Racket Exception Implementation
#lang racket
(define *handlers* (list))
(define (push-handler proc)
(set! *handlers* (cons proc *handlers*)))
(define (pop-handler)
(let ((h (car *handlers*)))
(set! *handlers* ( cdr *handlers*))
h))
(define (throw . x)
(if (pair? *handlers*)
(apply (pop-handler) x)
(apply error x)))
(define-syntax catch
(syntax-rules ()
((_ [handler ...] exp1 ...)
(call/cc (lambda (exit)
; ; install the handler
(push-handler (lambda (ex)
; ; rewrite the handler with a Pattern Matching Expression that rethrow the exception in the default case
(exit (match ex handler ... [_ (throw ex)]))))
(let ((res ; ; evaluate the body
(begin exp1 ...)))
; ok : discard the handler
(pop-handler)
res ))))))
(struct throwable ())
(struct exception throwable (message))
;; TESTS
;; redefine error to test throw...
(define (error . x) (apply displayln x))
;; Simple Test Case
(throw (exception "CiaoBao")) ;; Displays #<exception>
;; Simple Catch Test Case
(catch ([(exception "bao") (displayln "ciao")])
(throw (exception "bao"))) ;; Displays "ciao"
;; Not so simple Nested Catch Statement
(catch ([(exception "bao") (displayln "ciao")])
(catch ([(exception "ciao") (displayln "bao")]
[(throwable) (displayln "throwable")])
(throw (exception "bao")))) ;; Displays "throwable"
;; Simple Pattern Matching
(catch ([(exception _) (displayln "bao")])
(throw (exception "ciao"))) ;; Displays "bao"
;; Intermediate Pattern Matching
(catch ([(exception message) (displayln message)])
(throw (exception "Not so stupid message"))) ;; Displays "Not so stupid message"
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