On Fri, 5 Apr 2013, Dmitry A. Kazakov wrote:

>>> How is it an error to compare String and Wide_String?
>>
>> The same way it is an error to compare a float and an integer.
>
> It is not same. R and Z are different mathematical constructs.

Sure! This is different from string types, where the distinction between 
String, Wide_String, Wide_Wide_String, and whatever comes next has 
historical reasons. It makes mixing narrow, Wide_ and Wide_Wide_ Strings 
worse than mixing reals and integers.

> Unicode strings, all of them, are semantically exactly same except for 
> encoding [representation] and constraints put on the code point set.

N is a subset of Z is a subset of Q is a subset of R.

Furthermore, each "X is a subset of Y" relationship implies that any 
operation possible in X is also possible in Y, including comparisons.

How are generalised strings simpler than generalised numbers?

> Mixed operations are a must for all algebraic types and all string types.

Well, Ada does make a clear distinction between Universal_Integer and 
Universal_Real. So why should it work for String types?

> Ada 83 had it mixed from the start, e.g. Universal_Integer vs. Integer.

Ada (all Adas from 83 to 2012) had two *different* algebraic root types.

In any cases, there are good reasons why a program has sometimes to mix
real and natural numbers. But what reasons would you have to actually mix 
narrow, Wide_ and Wide_Wide_ Strings? That is bad program design, anyway!

It is error-prone low-level programming, quite similar to the C-style of 
using memory addresses and pointer arithmetic. If you *have* to do it, 
perhaps because you have to support some legacy interfaces, with different 
string types, you better hide the input/output stuff in some low-level 
packages, and use a single kind of string in your application logic 
(perhaps Wide_Wide_String, if the memory isn't too small).

Of course, a single common root type for String, Bounded_String and 
Unbounded_String would come handy -- similarly another root type for the 
Wide_ and yet another for the Wide_Wide_ family. But note that not even 
Universal_Integer does provide such a root type! The Standard(-Library) 
did not even define any Bounded_Integer or Unbounded_Integer types to deal 
with huge integers, in contrast to strings.




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