[r6rs-discuss] Raise-continuable and raise question

[Thomas Lord]

Ben Simon wrote:
>
> Thanks for the quick feedback.  Sorry to be so thick on this, but I'm 
> still not seeing it.
>

Oh, don't feel that way.   It's an "obvious after you see it (usually by 
being shown it) thing".


>
> When I write code that throws an exception, I don't think: "is this 
> continuable or not," I think "this has to fail, let someone else deal 
> with it."  I guess it boils down to the fact that I don't see why an 
> exception isn't always continuable depending on how clever the handler 
> wants to be.
>
> Perhaps you can provide an example of a continuable and 
> non-continuable circumstances?  That would probably clear things up.


Well, personally, I don't think dynamically scoped error handlers are a 
good idea at all and I think there's almost always a better way to code 
applications that want to use exceptions for non-error purposes.    So, 
you're asking me explain the design logic of features I don't think make 
a lot of sense -- but, hey, I can do that:

Example of continuable: Virtual Memory Page Faults!

One of the primitive operations in a typical OS kernel like Linux is 
"give some cycles to process X".   One of the exceptions that primitive 
can raise is "Oops -- X tried to access a virtual page of memory that 
isn't mapped!".      The kernel treats that as a continuable exception:  
it brings the demanded page into the virtual memory system and then 
retries the instruction that raised the exception.

and, example of non-continuable: Virtual Memory Illegal Address References

Illegal address references, on the other hand, don't continue (normally) 
--  they non-locally exit back to the parent process.

-t





>
> Thanks,
> Ben
>
> -- 
> Ben Simon
> My blog: http://benjisimon.blogspot.com
> tenspotting.com <http://tenspotting.com> - Top 10 Lists++ 

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