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
