Re: not null

[Jacob Sparre Andersen <sparre@nbi.dk>]

Adam Beneschan wrote:
> Hyman Rosen wrote:

> > [...] he's talking about having null references at all, so Ada is
> > no better.
> 
> I'm having difficulty understanding the point.  You have to have the
> idea of the lack of a reference.

> [...]  Suppose that access values in Ada were not allowed to have
> null values, and (for a linked list) you needed the concept of a
> value that may or may not refer to another object.  You could write
> this as a variant record:
> 
>    type Obj_Reference (Present : Boolean) is record
>       case Present is
>          when True =>
>             Ref : Obj_Access;  -- may not be null
>          when False =>
>             null;
>       end case;
>    end record;
> 
> I just don't see how this is an improvement; it simply shifts the
> problem.

No.

> Whatever mistakes a programmer could in the actual Ada language make
> by forgetting to check whether an access value is null, he could
> make in the above example by forgetting to check the Present
> discriminant.

No.  The _compiler_ checks the discriminant.  The programmer will get
a Constraint_Error if he tries to use "Ref" while "Present" is false.
And he can't change "Present" to true without also assigning "Ref" a
value.

Greetings,

Jacob
-- 
"For there are only two reasons why war is made against a
 republic: The one, to become lord over her: the other, the
 fear of being occupied by her."       -- Nicolo Machiavelli