Re: Making sense of predicates

[Jacob Sparre Andersen <jacob@jacob-sparre.dk>]

Shark8 wrote:

> On Tuesday, October 22, 2013 1:35:02 AM UTC-6, Jacob Sparre Andersen wrote:

>> package Parts is
>>    subtype Digit_Character is Character range '0' .. '9';
>>    subtype Part_Number is String
>>      with Dynamic_Predicate => (for all C of Part_Number
>>                                   => C in Digit_Character);
>> end Parts;
>
> I think you misunderstand my point: there's no real reason [at least
> that I can tell (apart from syntax, possibly)] that an array-type
> cannot be subtyped in order to place additional [static] constraint
> upon the base-type.

I've not tried to work out what differences there must be between how a
compiler implements dynamic and static predicates, but I assume that
there is a reason that the ARG decided to distinguish between the two
cases.

> IOW, why is the predicate required to be dynamic when, for example,
> "subtype Number_String is Array(Positive range <>) of Digit_Character"

That one isn't a subtype, which may hint at the reason.

> or "subtype short_String is String(1..255)" is not a
> dynamic-predicate?

A better example.  I'm not sure why, but I suspect that the
differentiation is due to how a compiler implements the checks.

If we can work out how to make the checks for some classes of
(currently) dynamic predicates as simple as those for static predicates,
we can probably get the ARG to reclassify those classes of predicates.

Greetings,

Jacob
-- 
"It ain't rocket science!"