The current exception handler really is determined by a continuation mark and not by a parameter (i.e., a mark that is separate from the one that determines the current parameterization).
At Fri, 3 Sep 2010 13:03:04 -0600, Jay McCarthy wrote: > That's my understanding of it too. [Maybe modulo parameters rather > than marks themselves.] > > Jay > > On Fri, Sep 3, 2010 at 12:05 PM, Danny Yoo <[email protected]> wrote: > > I'm about to try replicating Racket's exception system in WeScheme, so > > I want to make sure I understood the model beforehand. Here's my > > understanding of the system, and if people can correct me, I'd greatly > > appreciate it! > > > > > > * Exception handlers are managed by keeping a chain of these handlers > > within the continuation marks. A form like with-handlers effectively > > extends the chain within its dynamic extent. There's a designated > > continuation-marks key used to keep track of exception handlers. This > > key is internal and otherwise inaccessible to the outside world. Let > > me call this key "K" for the moment. If it were possible to get at K, > > then hypothetically I would be able to observe it by doing something > > like this: > > > > (begin > > (printf "Before: ~s~n" (continuation-mark-set->list > > (current-continuation-marks) K)) > > (with-handlers ([exn void]) > > (printf "Within: ~s~n" (continuation-mark-set->list > > (current-continuation-marks) K))) > > > > > > * Whenever an exception happens, the runtime immediately grabs the > > current continuation marks and extracts the chain of exception > > handlers associated to K. It then walks the chain. Each element in > > the chain is responsible for returning the exception up the chain. If > > an exception handler knows what to do with an exception, and wants to > > interrupt the chain, then it is supposed to call the > > error-escape-handler to stop the rest of the computation. > > > > > > I looked at the exceptions proposal [Friedman95 > > http://www.cs.indiana.edu/scheme-repository/doc.proposals.exceptions.html > > ]. I see that the proposal defines a current-exception-handlers, but > > I don't see such a function in Racket. I assume that the only way to > > adjust the current exception handler is through > > call-with-exception-handler. > > > > > > > > If I wanted to simulate the exception-handling mechanism, it might > > look something like this: > > > > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > ;;;;;;;;;;;;;;;;;;;;;;;;;; > > #lang racket > > > > (define K (cons "my-distinguished" > > "exception key!")) > > > > > > (define-syntax (simulate stx) > > (syntax-case stx () > > [(_ body ...) > > (syntax/loc stx > > (call-with-exception-handler my-exception-handler > > (lambda () > > body ...)))])) > > > > > > (define (my-uncaught-exception-handler exn) > > (printf "I am the uncaught exception handler, seeing ~s~n" exn) > > (abort-current-continuation (default-continuation-prompt-tag) void)) > > > > > > (define (my-exception-handler exn) > > (printf "my-exception-handler\n") > > (let ([handlers > > (continuation-mark-set->list (current-continuation-marks) > > K)]) > > (printf "I see the following handlers: ~s\n" handlers) > > (let ([exn > > (foldl (lambda (a-handler exn) > > (a-handler exn)) > > exn > > handlers)]) > > > > ;; At this point, calls the uncaught-exception-handler if none of the > > ;; handlers will support us. > > (printf "Uncaught exception handler kicking in.\n") > > (my-uncaught-exception-handler exn) > > (printf "I should never see this!\n")))) > > > > > > (define-syntax (my-with-handlers stx) > > (syntax-case stx () > > [(_ ([exn-test? on-exn] > > ...) > > body ...) > > (syntax/loc stx > > ;; NOTE: I need to make sure the with-continuation-mark is not > > in tail position, > > ;; or else my t2 shows that an exception handlers has been > > replaced, rather than nested. > > (let ([result > > (with-continuation-mark K (lambda (exn) > > (cond > > [(exn-test? exn) > > (on-exn exn) > > (abort-current-continuation > > > (default-continuation-prompt-tag) > > void)] > > ... > > [else > > exn])) > > > > (begin body ...))]) > > result))])) > > > > > > ;; test1: see that I can catch an exception > > (define (t1) > > (simulate (my-with-handlers ((exn:fail? (lambda (exn) > > (printf "Oh! I see ~s~n" exn)))) > > (printf "Starting up~n") > > (/ 1 0)))) > > > > ;; test2: see that I can observe the nesting, and also fire off the > > uncaught exception handler > > (define (t2) > > (simulate (my-with-handlers () > > (printf "Starting up~n") > > (my-with-handlers () > > (/ 1 0))))) > > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > > _________________________________________________ > > For list-related administrative tasks: > > http://lists.racket-lang.org/listinfo/users > > > > > > -- > Jay McCarthy <[email protected]> > Assistant Professor / Brigham Young University > http://teammccarthy.org/jay > > "The glory of God is Intelligence" - D&C 93 > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
