Re: Why doesn't Ada allow user-defined attributes?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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, should have no operations for it. 
Lots of external things cannot be, e.g. a file you have no permission to 
read. Lots of volatile things, e.g. a result set, a stream with its 
length in front of it etc, things you can traverse only once.

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