Re: Ada.Locales pseudo-string types

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

On 12-08-08 11:09 , Dmitry A. Kazakov wrote:
> On Wed, 08 Aug 2012 10:37:51 +0300, Niklas Holsti wrote:
> 
>> On 12-08-08 10:18 , Dmitry A. Kazakov wrote:
>>> On Wed, 08 Aug 2012 10:04:37 +0300, Niklas Holsti wrote:
>>>
>>>> The rule does seem surprisingly rigid, for a value conversion. (The
>>>> Annotated RM does not explain or motivate it.)
>>>
>>> Ada didn't appreciate structured type equivalence in earlier times. These
>>> rules were designed before anonymous access types crept in.
>>
>> But we are talking about explicitly requested type conversions, not
>> implicit type equivalence. As I understood it, at least.
> 
> Yes, but the validity of this conversion relies on an inferred equivalence
> of the [sub]types of the elements.

No, I would rather think that the explicitly requested array-type
conversion implies requests to convert the index [sub]types and the
element [sub]types, too.

> Ada considered them equivalent nominally.

Yes, but in order to be sure that the conversion requires no active
machine code and can be done just by relabeling the type of a reference
to the array.

> A less conservative approach is to consider only those for which
> constraints are no less permissive than ones of the target elements. 

Yes (I wrote about that in another reply).

>>>> For a value conversion,
>>>> the default could be to convert element by element, and the compiler
>>>> could then optimize this conversion based on what it can deduce
>>>> statically about the matching of the source and target subtypes.
>>>
>>> Could be interesting when elements themselves are arrays of discriminated
>>> record subtypes constrained via discriminants with array components again
>>> constrained etc.
>>
>> Sure, but this should not require much new intelligence in the compiler.
> 
> True.
> 
> BTW, there is another issue with that. What happens when some element
> cannot be converted? Constraint_Error is to propagate. What happens with
> the target. Will be previously assigned elements rolled back?

A matter of definition. Ada already contains the concept of a
"disrupted" assignment, which can be caused by the failure of a
language-defined check and in which the variable being assigned then
becomes abnormal.

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