Re: new revision ada

[Martin Krischik <krischik@users.sourceforge.net>]

James Rogers wrote:

> Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de> wrote in
> news:cb927q$332$1@a1-hrz.uni-duisburg.de:
> 
>> Ada has facilities to implement "trying-out programming". If one
>> (logical) thread of control fails, and it is important to
>> collect information about why it failed and in what program
>> state, than save the relevant pieces of the program state
>> using the executable statements of a local exception
>> handler for example.
>> You can store the information somewhere for lookup, and
>> use Ada's exceptions, or task communication, or plain
>> returned values to inform the caller of the failure or
>> success. The caller can use this information to retrieve
>> what has been stored for lookup.
> 
> Your solution detaches the information from the notification.
> This detachment causes problems, which are compounded by
> concurrency. If two exceptions are raised in a program and
> their state information is stored in some buffer such as
> a protected object, how will a handler determine which state
> belongs to which exception occurrence?

Store them in an booch component map indexed by thread id. I do that in
AdaCL.Trace.

> The state information must be directly associated with the
> exception occurrence. In Java this is done be creating an
> exception object. References to that object are passed
> up through the call stack until the exception is handled.

Posible only becase Java throws pointers only.

> I believe the same concept is used in C++ exceptions.

With the usual amount of pitfalls which acomplished every C++ feature - yes.

> Ada has a somewhat different view of exceptions. All exceptions
> are the same type, and are identified by name. Ada 95 added the
> ability to define exception information in a limited length
> string. That string is intimately associated with an exception
> occurrence. I am proposing that we provide a second kind of
> exception information using a variable-width string that
> encodes data in XML. XML is capable of encoding any kind of
> data including very complex data structures. This new form
> of exception information will allow complex exception state
> data to be reliably communicated with an exception
> occurrence.

Again: why XML when Ada has a perfectly usable system to stream out data.
i.E. 'Input and 'Output. Annex E is allready based on it so it could be
used for extended exceptions as well.

Martin

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