Re: Strange crash on custom iterator

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

On 2018-07-03 23:04, Lucretia wrote:
> On Tuesday, 3 July 2018 21:18:28 UTC+1, Dmitry A. Kazakov  wrote:
> 
>>> Well, they kind of already did that by subtyping UTF_String from String, of which it's not a subtype, it's just they are both arrays of 8-bit entities.
>>
>> No. Both are arrays of code points and arrays of octets. The ranges of
>> code points are different. The correspondence between code points and
>> octets are different. Thus the subtyping is broken.
> 
> I know the difference between code points and octets and their arrays. I was saying that UTF_String is not a valid subtype of String because String is Latin 1 and UTF_String is a superset of 7-bit ASCII, not 8-bit Latin 1.

No, that does not break subtyping if Constraint_Error is in the 
contract. Subtyping is broken when the array of Latin-1 code points 
(String) corresponds to the array of representation units (octets of 
UTF8_String).

Array of Latin-1 code points corresponds to the array of Unicode code 
points. It has nothing to do with the underlying encoding, whatever it 
might be.

Each string implements two unrelated array interfaces:

1. Array of encoding units, e.g. array of octets
2. Array of code points

#1 and #2 are historically confused because one resembles another for a 
certain class encodings like ASCII, UCS-2, UCS-4. They are absolutely 
different for UTF-8 and UTF-16.

>>> Am i wrong, should I just implement what I need on top of the standard lib and just use the UTF* types in my code? What about unbounded_utf_strings? Just use the normal unbounded_string? It's not like it's going to be checking for it to be correct utf8 is it, but I can't write an iterator for that from outside the rts though.
>>
>> There is no way to do it right in Ada for now.
> 
> What do you mean exactly????

For simplicity start with designing character types: Character, 
Wide_Character and Wide_Wide_Character as related types.

    X : Character; -- Character'Size = 8
    Y : Wide_Character := Y; -- This must be legal

Already this is impossible in Ada.

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