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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Handling transactions?
Date: Mon, 27 Jul 2015 19:10:09 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:27061
Date: 2015-07-27T19:10:09-05:00
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"EGarrulo" <egarrulo@gmail.com> wrote in message 
news:40c8dba8-85e9-42e7-8316-96c976531ab4@googlegroups.com...
>On Monday, July 27, 2015 at 6:38:37 PM UTC+2, David Botton wrote:
>> EGarrulo, your question is not a language question but a design question.
>> What you want can be done in any language and depending on what you
>> are trying to "roll back" I can think of many many ways to do it. Be 
>> specific
>> and I'm sure some of us can offer advice for your project.

>Thank you, but mine is a curiosity.  I am new to Ada and I am trying to 
>understand
>how to approach problems the Ada way.   My above hypothetical solution 
>mirrors
>what I would have done in C++, by following the RAII (Resource Acquisition 
>Is
>Initialization) idiom.  It seems to me that Ada offers a similar idiom with 
>its controlled types.

As always, the best approach depends on exactly what you're doing. In Claw, 
we needed exclusion locks in some cases, and we found the best pattern for 
them was to use a limited controlled type:

    type Lock is new Ada.Finalization.Limited_Controlled with null record;
    overriding
    procedure Initialize (Object : in out Lock);
    overriding
    procedure Finalize (Object : in out Lock);

The body of Initialize seizes the lock, and the body of Finalize frees the 
lock. Then, the simple act of declaring an object of type Lock gets the lock 
and holds it until the operation is complete (or fails); the lock is then 
freed (even if an exception is propagated).

This is typically used in a block:

    declare
         My_Lock : Lock; -- Gets lock.
    begin
         -- Do operations needing the lock.
    end; -- The lock is freed here, no matter how this block is completed.

I suspect that you could use a similar pattern for transactions (although 
I've never tried it).

                                   Randy.