Re: Discriminants of tagged types

["J-P. Rosen" <rosen@adalog.fr>]

Le 27/10/2010 17:58, Yannick Duchêne (Hibou57) a écrit :
> Indeed, this show very well how can default discriminants be a mess (and
> there are other reasons too to add to the ones given here).
> 
> A question then come : do someone know something to which default
> discriminants are unavoidable ? I feel default discriminants can simply
> be dropped (kind of sugar?), but I would like to be sure.
> 
This issue is very simple and understandable if you take it the other
way round.

Reminder:
I1 : Integer;  -- Can take all values of type Integer
I2 : Integer range 1..10; -- Constraint, restricts to value 1..10

with:
type Rec (D: Disc) is ....

I must declare:
R1: Rec (1); -- Constraint, restricts to value 1 of discriminant.

Wouldn't it be nice to have a variable that could hold all values of
type Rec? Naturally, this is written as:
R2: Rec;

But wait a minute. If the discriminant is not specified by the
declaration, what is its value? A discriminant is to serious to leave it
uninitialized. OK, therefore this construct is allowed only if the
discriminant has default values.

QED.

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           J-P. Rosen (rosen@adalog.fr)
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