Re: Ada.Locales pseudo-string types

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

On 12-08-07 23:42 , Marius Amado-Alves wrote:
>>>>>    subtype Country_Code is String (1 .. 2) 
>>>>>       with Static_Predicate =>
>>>>>          Country_Code (1) in 'A' .. 'Z' and
>>>>>          Country_Code (2) in 'A' .. 'Z';
>>
>>> If Country_Code denotes the object (?), and the object is a String, then I 
>>> don't see a problem in indexing it.
>>
>> I don't know what you mean by "you don't see a problem". 
>> There are (I'm sure) places in the language where a "current
>> instance" of an array type can be indexed, but a static predicate
>> isn't one of them.
> 
> Dynamic, then.
> 
> But this is hardly useful. It has the same component-by-component ugliness as
> 
>    String'(1 => Country (1), 2 => Country (2))
> 
> The static match rule for component types is silly. It should
> be a compatibility match like it is for the main types. At least.

The rule does seem surprisingly rigid, for a value conversion. (The
Annotated RM does not explain or motivate it.) 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.

In the example, the compiler should be able to deduce that
element-by-element conversion is unnecessary since the source element
subtype is a subrange of the target element subtype.

> Ada has way too strict rules for conversion. As a consequence Ada
> programs are ridden with Unchecked_Conversion instances.

Depends. In my main Ada product, in size about 77,000 semicolons, there
is not a single use of Unchecked_Conversion. Just one data point, of course.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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