From: David Simmons <pulsar@...>
Date: 2001-07-19T23:41:10+09:00
Subject: [ruby-talk:18121] Re: Exceptions

"Yukihiro Matsumoto" <matz@ruby-lang.org> wrote in message
news:iAx57.28006$PF1.1401786@e420r-atl2.usenetserver.com...
> Hi,
>
> In message "[ruby-talk:18102] Re: Exceptions"
>     on 01/07/19, "Aleksei Guzev" <aleksei.guzev@bigfoot.com> writes:
>
> |Can one implement such "resume" without changing the ruby core itself?
>
> No.  It requires deep core rewrite.  Not an easy task at all.
>
> matz.
>

If I understand it correctly, Ruby provides real/complete block closures?

If so, you might find it useful to review the 1998 ANSI Smalltalk Standard.
The rich exception model it provides can all be built in pure Smalltalk code
based on using block closures.

If Ruby has real/complete block closures, someone should be able to provide
a full featured set of portable exceptions by writing a library directly in
Ruby code itself.

The Smalltalk exception model supports:

ACTIONS WITHIN A GUARD/FILTER:
-----------------------------
pass - don't handle the exception

resume - continue execution from the exception signalling expression.
Optionally allows replacing the return value.

return - unwind intervening guards and exit from this guard. Optionally
allows replacing the return value.

outer - calculate the result/status of invoking an outer guard. Allows an
inner guard to call outer guards while leaving the inner guard in control of
what happens next.

resignalAs - restart the exception handling from the signalling expression
but substitute and alternate exception.

retry - exit the currently guarded context and retry the execution of that
context (the try block).

retryUsing - exit the currently guarded context and retry using the supplied
block [within the current guard context] instead of the original "try"
block.

break - is equivalent to "return:"

Other useful items that can be queried are:

    isCurtailed
    isException
    isSignal
    isResumable
    isNested

    signalValue

    signallingContext [from which you can walk all the intermediate frames]
    unwindContext

    walk the nested (outer) guard contexts, etc.

Various implementations, like my work with QKS Smalltalk (and SmallScript)
actually have a richer model than that which is provided by the ANSI
standard [because the standard had to be a common and easily implementable
subset of available implementation features such as those which were in QKS
Smalltalk at the time the standard was written]. My work actually is unified
with C++ (and SEH) facilities on its various platforms, and directly into
the .NET exception system on the .NET platform.

Based on my experiences, it is only moderately hard work to implement all
the above features as essentially zero cost exception handling. Where all
the "blocks" actually become inlined subroutines (since none of them are
actually capturable). The key is to integrate it directly into the non-local
return [unwind] semantics of the languages execution engine (VM).

--
-- Dave S. [ http://www.smallscript.net ]