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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: A function that cannot be called?
Date: Fri, 19 Oct 2018 21:57:39 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:54662
Date: 2018-10-19T21:57:39+02:00
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On 19.10.18 19:06, AdaMagica wrote:
> I do not really understand what all this is about, but:
> 
> package What is
>    type Void (<>) is private;
>    function Create (Params: Some_Type) return Void is abstract;
> private
>    type Void is null record;  -- no values
> end What;
> 
> Void has no values (hm, a null record is a value, but it's irrelevant, since no objects exist (there is none in the private part; a body is illegal); no objects can be created because Create is abstract, so in effect Create does not exist - so it cannot be called.


    41.     Consume (What.Create'Access);
                      |
         >>> prefix of "Access" attribute cannot be abstract

If the above package is placed in some private part, then, I think,
the language will guarantee that there is no way of introducing
objects anywhere.

 From the docs of Haskell's Data.Void:

"A Haskell 98 logically uninhabited data type,
  used to indicate that a given term should not exist."


One use of all this could be: any attempt at calling
the access-value passed to Consume will trigger some kind of
exceptional situation.
This implement works when Void is range 1..0 such that objects
can be declared, but is not available with either abstract
functions, as seen, or with a Void(<>) that cannot be constrained.