Re: About exception handling again ...

[John Cowan <cowan@ccil.org>]

On Sun, Aug 2, 2020 at 2:05 PM Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>
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 <
https://github.com/johnwcowan/r7rs-work/blob/master/RestartsCowan.md>.  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          http://vrici.lojban.org/~cowan        cowan@ccil.org
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)