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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How to get Ada to ?cross the chasm??
Date: Wed, 9 May 2018 22:41:55 +0300
Organization: Tidorum Ltd
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Xref: reader02.eternal-september.org comp.lang.ada:52165
Date: 2018-05-09T22:41:55+03:00
List-Id: <comp.lang.ada>

On 18-05-09 00:46 , Dmitry A. Kazakov wrote:
> On 2018-05-08 23:28, Niklas Holsti wrote:
>> On 18-05-07 10:25 , Dmitry A. Kazakov wrote:
>>> On 07/05/2018 03:31, Paul Rubin wrote:
>>>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>>>>> This is easily done in present Ada using handles to a
>>>>> reference-counted objects.
>>>>
>>>> That's not much different from gc from the user's perspective, other
>>>> than problems handling cycles.
>>>
>>> Cycles are easily avoidable too by separating target objects and
>>> handles. All target objects types go into the private part of the
>>> package. When only handles are visible there is no way the end user
>>> could construct a cycle.
>>
>> True for a single such data structure; much harder if the client
>> combines different, independently implemented data structures and
>> stores, in one structure, handles that point at the other structure,
>> and perhaps in the other direction too. Since the implementations of
>> the two data structures are ignorant of each other's existence,
>> neither can detect such cross-structure cycles, I believe.
>
> Yes, but that is a bad design anyway. I would say that all such cases
> fall into the category when references going in one direction are strong
> and the backward ones are weak.

That makes it difficult to maintain value semantics for these data 
structures; how could one have "weak" and "strong" values?

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .