Hi! On 7/12/20 6:08 PM, Catonano wrote: > Il giorno sab 11 lug 2020 alle ore 12:14 Chris Vine <vine35792...@gmail.com> > ha scritto: > >> On Sat, 11 Jul 2020 02:19:43 +0200 >> Zelphir Kaltstahl <zelphirkaltst...@posteo.de> wrote: >> [snip] >>> I would be glad, if any non-optimal example was extended or updated by a >>> more knowledgeable person or I was told what I could improve in some >>> example. The examples in the repository are only, what I was able to >>> understand and I often find myself looking up, how to do something again. >>> >>> If anyone wants to take any example and put it in the docs, go ahead. >>> Only don't think that my examples are the last word on how to do things. >>> Far from it! >>> >>> About the exceptions thing: Aha! I should look into that again. I >>> thought the "conditions" thing was already pretty cool and useful. Once >>> an exception happens, you can react on it. I did not understand the >>> "call/ec helper" part, but perhaps I can understand it, when I check the >>> docs for the new exceptions in Guile 3. >> The issue is that "non-continuable" in "non-continuable exception" does >> not mean an exception that the program cannot survive, it means an >> exception for which, after handling, control cannot return to the point >> at which the exception was raised[1]. (To answer the question in your >> following email, continuable exceptions are in some sense analogous to >> common lisp restarts.) Most guile exceptions are non-continuable. The >> point arising from this is that in the case of a non-continuable >> exception the handler procedure passed to with-exception-handler must >> not return, or a &non-continuable exception will be raised when >> control does attempt to return. >> >> With R6RS/R7RS's with-exception-handler, for non-continuable exceptions >> the handler procedure should normally therefore either invoke a call/ec >> continuation object to unwind the stack to the point where the >> exception is handled, or it should (after it has done what it is >> intended to do) re-raise the exception to be handled and/or unwound >> elsewhere. Guile-3.0's with-exception-handler procedure will do the >> former for you automatically if you set its #:unwind? argument to >> true. The nearest to guile-2.2's catch expression in guile-3.0's >> exception system is to use with-exception-handler with #:unwind? set >> as #t. R6RS/R7RS's guard form is a wrapper for this which also >> incorporates a cond form to enable different exception types to be >> handled by different handlers. >> >> I therefore suggest that your first example using >> with-exception-handler should set #:unwind? to #t so that the program >> does not end with a &non-continuable exception. I also suggest that, >> if intended for guile-3.0 and not guile-2.2, you should use guile's >> exception objects rather than R6RS conditions (basically you use >> 'make-exception' instead of 'condition' - the two are in effect the >> same). >> > > If it can be of any help, I applied your suggestions to the original > example > > This is supposed to be compatible with Guile 3.x only > > Here it is (a question follows) > > (define even-simpler-display > (lambda (sth) > (display > (simple-format > #f "~a\n" sth)))) > > > (define divide > (lambda (a b) > (cond > [(= b 0) > (raise-continuable > (make-exception > (make-exception-with-message "division by zero") > (make-exception-with-irritants (list a b)) > (make-exception-with-origin 'divide)))] > [else > (/ a b)]))) > > > (with-exception-handler > (lambda (conditions-or-value) > (even-simpler-display > ;; We can get the simple exceptions from a compound exception with > the > ;; `simple-exceptions` getter. > (simple-exceptions conditions-or-value))) > (lambda () > (divide 2 0)) > #:unwind? #t) > > I run this in the REPL and it seems to work > > The question, now, is: > > say that the exception handler knows (somehow) that the correct divisor is > 1 rather than 0 > > So we are saying that this exception is continuable > > How would we continue ? > > Can "divide" return as if it had been called with 1 as its second argument ? > > Or have I misunderstood ? > > How would you go about that ?
Ah, I started putting "to do changes" in a list to apply them later, with better understanding: https://notabug.org/ZelphirKaltstahl/guile-examples/src/d670fa02369918cd3fa896cc19d5aa82966f64a3/exception-handling/notes.org I was going to come back to points, if I do not understand anything properly. Perhaps I can already copy some parts from here^^ Thanks! Zelphir
