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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Why doesn't Ada allow user-defined attributes?
Date: Sat, 22 Jul 2017 18:33:22 +0200
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Date: 2017-07-22T18:33:22+02:00
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On 21.07.17 21:39, Dmitry A. Kazakov wrote:
> On 2017-07-21 19:11, Georg Bauhaus wrote:
>> Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>>> On 21/07/2017 06:46, Georg Bauhaus wrote:
>>>
>>>> Do you really need 'Length?
>>>
>>> Sure, when for example the result depends on the number of elements.
>>
>> Count is computed easily, albeit inefficiently, or even efficiently
>> whenever a collection type caches its number of elements.
>> So, is 'Length, if not a static property, more than a prim op?
>>
>>> 'Length is a general property of any container (set).
>>>
>>>> It seems an implementation detail whenever the language doesn't have forall
>>>> or exists.
>>>
>>> Enumeration requires some additional assumptions, stronger than length.
>>
>> What real thing can be counted yet not one by one?
>
> Unordered set cannot be enumerated,

https://en.wikipedia.org/wiki/Enumeration

> Lots of external things cannot be, e.g. a file you have no permission to read.

A file needs to have been _counted_ byte by byte,
or allocated using a known _count_ of bytes, etc.
in order to have this length.
Permissions don't change this state of affairs.