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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: A function that cannot be called?
Date: Thu, 18 Oct 2018 22:21:10 +0200
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Date: 2018-10-18T22:21:10+02:00
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On 18.10.18 17:33, Stefan.Lucks@uni-weimar.de wrote:
> On Thu, 18 Oct 2018, G.B. wrote:
> 
>> There is a type declaration, I rememberd, in Ada, that
>> does not allow any object of the type to be declared.
> 
> What, exactly, is the point of your question?

Philip Wadler asked, on occasion[*], whether there was any programming
language other than Haskell that had a (concrete, usable in the normal
way I guess) type like ("unihabited") Void in Data.Void.  (The point is
getting my feet wet at the shores of correspondences of logic, typed lambdas,
and categories with resp. programming languages' features.)

As a minimum, I imagine this means passing access-to-function values,
each such value designating a function whose signature include type Void.
Abstract functions cannot be designated, IINM, i.e., no 'Access.


> [... abstract ...]
> Note that objects of type Void'class can exist, but never, ever, any objects of type Void. Thus, for good reason the Ada compiler refuses to compile
> 
>    function Impossible(X: T) return Void;


So, I do want the above function to exist.
I'll put the following one on the shelf, then:

     function Impossible (X : T) return Void is abstract;

Although, the idea is, presumably, that I can use an access-to-function
in some ways, but not actually call (apply) it or one of its overriding
substitutes. All of this, I think, an Ada compiler will prevent when
type Void is part of the signature.

__
[*] Propositions as Types, Talk at Lambda Days 2016 (according
to youtube)