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Re: About exception handling again ...

From: Zelphir Kaltstahl
Subject: Re: About exception handling again ...
Date: Tue, 4 Aug 2020 01:17:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Hello John!

On 8/3/20 6:41 AM, John Cowan wrote:
> On Sun, Aug 2, 2020 at 2:05 PM Zelphir Kaltstahl
> < <>> wrote:
>     1. Is there any situation, in which one would like to raise a
>     non-continuable exception / condition, and not unwind the stack? Would
>     that make sense in any situation?
> I'm going to talk about Scheme in general, not Guile specifically. 
> There are at least three use cases:
> 1) A reflective low-level debugger that can examine variables in the
> current stack frame, which can't possibly work if the stack frame is gone.
> 2) A handler that wants to examine the dynamic environment in place
> when the exception was raised.
> 3) A restart system that returns to the context of the raise and
> executes recovery code, often chosen on the basis of interaction. 
> Frequently used recovery actions include retrying the failed operation
> (on the assumption that the external environment has changed),
> retrying the operation with one or more different variables, and
> setting local variables to new values and retrying.
> A "Lisp debugger" is code that examines the available restarts and
> invokes one of them, but Scheme doesn't have such a thing yet because
> it has no restart system yet.  See
> <>. 
> CL has a restart system rather more complex than my proposal, which is
> based on an earlier proposal by Taylor Campbell.
>     Is this all correct?
> I would say "correct but not complete".
>     3. What would be a code example for a continuable exception /
>     condition
>     and what does the "continuing" there look like? I think the idea of
>     exception in my head is still influenced a lot by Python or other
>     languages, where it seems like all exceptions are non-continuable. (Is
>     that correct?)
> Let's say a procedure wants to open a configuration file, but the
> configuration file cannot be opened.  Raising a continuable exception
> allows the handler to take some recovery action (perhaps it prompts
> the user for the location of the config file) and return to the point
> where the exception was raised.  Restarts extend this concept by
> providing a protocol for the handler to communicate with the raiser,
> just as condition objects are a protocol to let the raiser communicate
> with the handler.
> In languages without continuable exceptions, such a retry must be done
> from the handler level, which may imply re-executing a lot of code
> that was run before the config file was wanted.  With continuable
> exceptions, this is not necessary.
> The `guard` special form basically emulates the behavior of {Python,
> JS, Java, C#} style exception systems in cases where that behavior is
> sufficient.
>     4. Is there a situation, where one would like to raise a continuable
>     exception / condition, but also unwind the stack?
> Not that I can think of.
>     IWhat does it mean for with-exception-handler to "return"? How can
>     it not
>     return? Does this mean CPS like not returning, or does it mean "not
>     return a value"?
> The former.  An exception being raised non-continuably is really
> raised continuably, but then if that returns, an exception that means
> "attempt to continue from a non-continuable exception" is raised
> non-continuably.  If you aren't careful in your handler to re-raise
> exceptions you don't understand (normally with raise-continuable),
> this will obviously get into an infinite loop.
> (In CL the behavior of a handler is slightly different.  If it
> returns, the exception is re-raised automatically. so in order to
> unwind the stack one must use one of CL's three upward continuations: 
> block/return, tagbody/go, or catch/throw.  The last is dynamically
> scoped, so it is usually the thing to do in this context. 
> Handler-bind, like guard, is a convenience macro for emulating
> stack-unwonding systems._
> John Cowan      
> <>
> He that would foil me must use such weapons as I do, for I have not
> fed my readers with straw, neither will I be confuted with stubble.
>                         --Thomas Vaughan (1650)
Thank you for the examples! They explain, why one would want to have the
flexibility to decide whether to unwind or not to unwind.

I'll try to perhaps add a phrase like "There are some scenarios, in
which not unwinding makes sense, for example …".

I'm glad, that my understanding was not completely wrong.


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