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 ?