Re: "Missing" libraries/concepts found in other languages/ecosystems?

[Zelphir Kaltstahl]

Hi!

On 7/12/20 6:08 PM, Catonano wrote:
> Il giorno sab 11 lug 2020 alle ore 12:14 Chris Vine <vine35792468@gmail.com>
> ha scritto:
>
>> On Sat, 11 Jul 2020 02:19:43 +0200
>> Zelphir Kaltstahl <zelphirkaltstahl@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