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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: Sun, 23 Jul 2017 08:41:40 +0200
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Date: 2017-07-23T08:41:40+02:00
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On 22.07.17 22:12, Dmitry A. Kazakov wrote:
> On 2017-07-22 18:33, G.B. wrote:
>> 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
>
> The sixth word there.

A set's listing is different from the set---it's a name.
But a count of a set's listing also happens to return
the set's number of elements.
An order established by counting is not a property of the set it self,
but may involve listing it somehow. A specific ordering is not needed
for counting.

>>> 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.
>
> No. File length is an attribute of a file. You don't need to read file or to allocate it.

Someone or something must have established the count by,
well, counting. Otherwise no file can meaningfully have
that attribute (length) in the first place!