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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Tagged type more type safe than access to subprogram ?
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Date: Fri, 30 Oct 2009 09:25:12 +0100
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Date: 2009-10-30T09:25:09+01:00
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On Thu, 29 Oct 2009 21:45:02 +0100, Georg Bauhaus wrote:

> Dmitry A. Kazakov schrieb:
>> On Thu, 29 Oct 2009 04:00:29 -0700 (PDT), Hibou57 (Yannick Duchêne) wrote:
>> 
>>> On 28 oct, 09:55, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
>>> wrote:
>>>> An access to subprogram is a poor-man's closure. Let's ignore "access" part
>>>> and consider a pure downward closure (as it should have been in Ada).
>> 
>>> “ As this should have been in Ada ” ? What were you to mean ? I've
>>> always though real closures are not possible with such structures as
>>> Ada provides, except at package level — which is especially the case
>>> when a package can have multiple instances... but only at package
>>> level. Isn't it ?
>> 
>> I meant downward closures. [...]
> 
>> BTW, in Ada 83, there waa no access to subprogram, so we used tasks instead
>> (where a subprogram had to be a non-generic parameter).
> 
> Tagged types may come close to a solution sometimes.

Yes, I am using this pattern very often. But:

1. There were no tagged types in Ada 83

2. In Ada 95 tagged types were in effect strictly library-level. This
restriction was lifted only in Ada 2005, too late to stop the "access-to
everything cancer".

3. The language is too heavy when it comes to create a singleton object
overriding one or two abstract primitive operations. There should be
short-cuts for that.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de