Re: Ada Annoyances

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-06-24 22:55 , Simon Wright wrote:
> Niklas Holsti <niklas.holsti@tidorum.invalid> writes:
>
>> For some time, I have had in mind a possible Ada extension to solve
>> point (b): an attribute/aspect that would let the programmer set a
>> static upper bound on the size of any object in T'Class. If we call
>> this aspect Maximum_Size (or perhaps Maximum_Size'Class), the
>> programmer could use it like this:
>>
>>    type Root is tagged record ... end record
>>    with Maximum_Size => 128;
>>
>>    type Child is new Root with record ... end record;
>>    -- The compiler checks that Child'Size is at most 128 bits, and
>>    -- rejects the program otherwise.
>>
>> It would now be legal to create statically sized data structures using
>> Root'Class, without dynamic memory allocation, by allocating 128 bits
>> for each value of type Root'Class:
>
> GNAT is happy with
>
>    type Parent is tagged null record
>    with Dynamic_Predicate => Size (Parent) < 128;
>
>    function Size (P : Parent'Class) return Integer is (P'Size);
>
>    type Large is array (1 .. 10) of Integer;
>    type Child is new Parent with record
>       L : Large;
>    end record;
>
> Declaring an object of type Child raises Assert_Failure.

As one would expect, based on standard Ada, yes?

> Of course you'd much rather have a static compile-time check!

Indeed I would.

But the check is not the main point in the suggested Maximum_Size 
aspect: the main point is that it would let the compiler consider the 
type Root'Class as a definite subtype, and would therefore allow its 
direct use as a component of arrays or records, instead of forcing an 
access-classwide to be used as an intermediate.

I don't suppose GNAT lets you use Parent'Class as the component type of 
an array, even with this Dynamic_Predicate?

There may however be some other semantic implications of the definite vs 
indefinite subtype divide, not related to the size of the values, that 
would make it hard to let the suggested Maximum_Size aspect change the 
classwide type from indefinite to definite.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .