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Date: Fri, 06 Jun 2014 19:51:33 +0200
From: "G.B." <rm-dash-bau-haus@dash.futureapps.de>
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Subject: Re: a new language, designed for safety !
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Date: 2014-06-06T19:51:32+02:00
List-Id: <comp.lang.ada>

On 06.06.14 17:13, Dan'l Miller wrote:
> If a fatal runtime error due to an unimplemented method (which Ada would catch at compile-time, but Swift & Objective-C would catch only at runtime via the equivalent of an exception whose default behavior is to stop executing the entire program), then if that fatal runtime error causes any sort of dangerous mishap (e.g., near miss; crash; collision; fire) due to the software no longer performing its function, then the NTSB and/or FAA rightly would identify that fatal runtime exception and the lack of method-implementation that Ada (and C++ and a very few other languages) would have caught at compile-time as the root-causes of the dangerous mishap.

Since Ada programs may well have "unimplemented methods",
I think this appraisal of Ada's capabilities needs to be
augmented a little in order to be valid, viz. by referring
to reversed defaults in the way dispatching is part of the
language, and to rule checkers.
   Because, with regard to dispatching, if an Ada programmer
decides to use run-time dispatching, then there may well be
run-time errors. These include calling an undefined primitive
operation of a tagged type. This is demonstrated below.

The program fails predictably, even though it does not use
pointers or tricks.

$ ./comptime

raised CONSTRAINT_ERROR : comptime.ada:35 tag check failed
$

That's a run-time error, I'd say, stopping the program.
The state of P2.Op being unimplemented is announced in source,
though, ("abstract") which is different from just absence
as in Objective-C etc.

Consequently, a set of additional rules is needed to prevent
such errors in Ada , and some checking as can be performed
with AdaControl, I should think. Ada alone is better if less
lenience is, but won't suffice.

package Pak is

    pragma Pure (Pak);

    package P1 is
       type A is abstract tagged private;
       procedure Op (X : in out A) is abstract;
    private
       type A is abstract tagged null record;
    end P1;

    package P2 is
       type T is new P1.A with private;
       overriding procedure Op (X : in out T);
    private
       type T is new P1.A with null record;
    end P2;

    package P3 is
       -- Drills a hole!
       type TA is abstract new P2.T with private;
       overriding procedure Op (X : in out TA) is abstract;
    private
       type TA is abstract new P2.T with null record;
    end P3;

end Pak;

with Pak;
procedure Comptime
is
    Obj : Pak.P2.T;
    X : Pak.P1.A'Class := Obj;
begin
    Pak.P3.TA'Class(X).Op;  -- run-time dispatching
end Comptime;

package body Pak is

    package body P2 is
       overriding procedure Op (X : in out T)
       is begin
          null;
       end Op;
    end P2;

end Pak;