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Re: GNU Guile 2.9.5 Released [beta]

From: Chris Vine
Subject: Re: GNU Guile 2.9.5 Released [beta]
Date: Mon, 6 Jan 2020 23:14:19 +0000

On Mon, 06 Jan 2020 21:34:59 +0100
Andy Wingo <address@hidden> wrote:
> On Mon 06 Jan 2020 00:26, Chris Vine <address@hidden> writes:
> > I have a 'try' macro which adopts the approach that if an exception
> > arises, the macro unwinds from the dynamic environment of the code
> > where the exception arose to the dynamic environment of the call to
> > 'try', evaluates the cond clauses in that environment, and then if no
> > cond clause matches re-raises the exception in that environment with
> > 'raise' (rather than 'raise-continuable').  In other words, it does
> > stack unwinding in the same way as exception implementations in almost
> > all other mainstream languages which use exceptions.  It would be
> > trivial to implement this with guile-3.0's with-exception-handler with
> > its unwind? argument set to true.
> I am not sure this really matches with this use case:
>   (define (call-with-backtrace thunk)
>     (call/ec
>      (lambda (ret)
>        (with-exception-handler
>          (lambda (exn)
>            (show-backtrace exn) ;; placeholder
>            (ret))
>          thunk))))
>   (define (false-on-file-errors thunk)
>     (call/ec
>      (lambda (ret)
>        (with-exception-handler
>          (lambda (exn)
>            (if (file-error? exn)
>                (ret #f)
>                (raise-continuable exn)))
>          thunk))))
>   (define (foo f)
>     (call-with-backtrace
>      (lambda ()
>        (false-on-file-errors f))))
> If there's an error while invoking `f' that's not a file error, you want
> to have remained in the context of the error so you can show a full
> backtrace.  To my mind this is central to the exception handler design.
> So far so good I think.
> If I change the implementation of `false-on-file-errors' to be:
>   (define (false-on-file-errors thunk)
>     (guard (exn ((file-error? exn) #f))
>       (thunk)))
> I think this change should preserve the not-unwinding environment that
> `call-with-backtrace' expects.

Good point.  My approach does provide the programmer with less conveyed
stack information after the re-raise of an unhandled exception,
requiring more manual intervention to recover the information when
debugging the exception.

Before you suggested it I had not previously considered your proposal.
It may turn out to be the optimum solution, but I wonder if it would
surprise the programmer to have the cond conditionals evaluated in a
different dynamic environment from the one in which the cond
consequential is evaluated where there is a conditional which is true.
But I am not sure if that is of any importance.


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