Re: Generic procedures and their parameters

[Maciej Sobczak <no.spam@no.spam.com>]

Dmitry A. Kazakov wrote:

>> In C++ the fact 
>> that int is ordered does not require that it relates to some Ordered 
>> class.
> 
> Which is unfortunate for both C++ and Ada, because "to be ordered" = "a
> member of Ordered class." There is no obvious reason why they should be
> different.

No obvious, but subtle, yes.
Integer is Ordered, but so is String. Can you imagine a 
Sequence_Of_Ordered containing, say, 7, 123, "Dmitry", -5, "Maciej"?
All these objects are Ordered. Just like Banana is Fruit and Apple is 
Fruit, right? Still, I can imagine a container of Fruits with Bananas 
mixed with Apples, but certainly not a container of Ordered - and the 
problem is that the values listed above don't really share much in 
common. Can you sort a Sequence_Of_Ordered? No.

So - what does it really mean that Integer and String are both Ordered?

OK, I know - we forgot that their "domains" of ordering are disjoint.

But then - what's the point of having Ordered'Class?

As far as I perceive it, the fact that Banana is Fruit and the fact that 
Integer is Ordered are two facts of *different kind*. The former is 
related to the "business disctionary" (nice buzz), whereas the latter is 
more an information about the type. For me, these two facts exist on 
separate levels and should not be mixed in the same inheritance 
hierarchy. Moreover, the language should give me distinct tools to 
express these distinct kinds of facts and for example in C++ I use class 
inheritance for Banana is Fruit, but type traits for Integer is Ordered. 
If I had to express these two in the same way (and in the same 
hierarchy) I would feel a bit uncomfortable. Kind of "weak type safety 
on the meta-level".

There is another "small issue". Robert mentioned that trees are 
aesthetically pleasing. The problem is that with Ordered (Comparable, 
Summable, etc.) there is no tree any more because of massive 
proliferation of multiple inheritance that is then used to express facts 
about types. The only "pleasing" thing left in the resulting mess is 
that there are still no cycles. ;-)


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/