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

[Catonano <catonano@gmail.com>]

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 ?