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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: A function that cannot be called?
Date: Sun, 21 Oct 2018 12:14:03 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:54679
Date: 2018-10-21T12:14:03+02:00
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On 20.10.18 00:06, Jere wrote:

> Are you ok with runtime checks?  If so, you can create a separate package
> for the void type:
> 
>      package Voids is
>          type Void is limited private;
>      private
>          type Void is new Ada.Finalization.Limited_Controlled
>              with null record;
>          overriding procedure Initialize(Self : in out Void);
>      end Voids;
>      package body Voids is
>          procedure Initialize(Self : in out Void) is
>          begin
>              raise Program_Error;
>          end Initialize;
>      end Voids;
> 
> Here, if try to create a variable of type Void, it will raise
> Program_Error (or you can add a custom exception.  Same for if
> you call a function that tries to return a Voids.Void variable.

I like the idea because the compiler will probably not suppress
raising the exception. This is unlike relying on Constraint_Error
being raised for 1..0. The not null constraint on a component
triggers C_E not matter what, it seems. However, Initialize states
the intention directly.