Re: Ada.Locales pseudo-string types

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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. Ada considered them equivalent
nominally. A less conservative approach is to consider only those for which
constraints are no less permissive than ones of the target elements. The
most lax would be to allow any subtype.

>>> 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?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de