About exception handling again ...

[Zelphir Kaltstahl]

Hello Guile Users!

In order to update the exception handling examples in my examples
repository
(https://notabug.org/ZelphirKaltstahl/guile-examples/src/master/exception-handling),
I've spend some time thinking about exception handling, what was written
on the mailing list and some things I read in the reference manual. I
think I understand a bit better now, but I still have some question
marks in my head:

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?

2. My rationale for unwinding the stack is written in the comments in
the example code:

~~~~
  ;; From the Guile reference manual:

  ;; "[...] it’s often the case that one would like to handle an exception by
  ;; unwinding the computation to an earlier state and running the error handler
  ;; there. After all, unless the raise-exception call is continuable, the
  ;; exception handler needs to abort the continuation. To support this use
  ;; case, if with-exception-handler was invoked with #:unwind? #t is true,
  ;; raise-exception will first unwind the stack by invoking an escape
  ;; continuation (see call/ec), and then invoke the handler with the
  ;; continuation of the with-exception-handler call." --
  ;; 
https://www.gnu.org/software/guile/docs/master/guile.html/Raising-and-Handling-Exceptions.html

  ;; Here the exception is non-continuable, so according to the reference
  ;; manual, we need to call with-exception-handler with #:unwind? being set to
  ;; #t for normal exception handling. If we do not do this, a (different?)
  ;; non-continuable exception is raised.

  ;; Q: Why unwind the stack?

  ;; A:

  ;; If the execution cannot be continued (non-continuable exception) at the
  ;; point, where the exception is raised, it means, that in that context, there
  ;; was insufficient information to continue the execution in useful way.

  ;; Unwinding the stack means discarding stack frames of procedure calls. This
  ;; happens up to the point, where an exception handler is defined. The result
  ;; of unwinding the stack is, that we only leave intact the environment, which
  ;; was available when the exception handler was defined.

  ;; We have the knowledge at this point, that an exception occurred. That is
  ;; more than we knew, when we called the procedure, which raised the
  ;; exception. Furthermore the exception can contain information about the kind
  ;; of thing that went wrong. This information hand-over facility should be
  ;; used to give sufficient information to the exception handler, so that the
  ;; handler can continue execution in a useful way. The exception enables us to
  ;; store important information from the environment at the point where the
  ;; exception was raised. Then we have no need to keep the environment of the
  ;; point, where the exception was raised and can unwind the stack.
~~~~

Is this all correct?

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?)

4. Is there a situation, where one would like to raise a continuable
exception / condition, but also unwind the stack?

5. Are the updated examples correct usage of exception handling?

6. Why is it recommended to make use of (make-exception ...) instead of
the rnrs facilities in Guile? Wouldn't using rnrs stuff be more portable
to other Schemes?

7. I noted from the last discussion the following:

~~~~
In case of "non-continuable" exceptions the handler procedure given to
~with-exception-handler~ should not return. If it returns, then another
"non-continuable" exception will be raised when it tries to return.
~~~~

What 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"?

8. What would I need to do to the current updated examples, to make good
Guile 2.2.x examples?

Regards,
Zelphir

-- 
repositories: https://notabug.org/ZelphirKaltstahl