Re: Operation can be dispatching in only one type

[Robert A Duff <bobduff@shell01.TheWorld.com>]

stefan-lucks@see-the.signature writes:

> On Mon, 7 Dec 2009, Randy Brukardt wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
>
>> > Why not to allow such constraints for subtypes? E.g.
>> >
>> >   subtype Valid_Window_Type is Window_Type when Is_Valid;
>> >
>> > then simply:
>> >
>> >   procedure Do_Something (Window : Valid_Window_Type; ...)
>> 
>> That's also under consideration, 
>
> Great! 

Yes, it is great.  I am a strong advocate of this feature.
But there are a lot of details to work out, and the feature
might not make it into the language by 2012.  We'll see.

> Isn't that essentially the same as the invariants in Eiffel?

Yes, it's quite similar.  But there are some important differences,
such as the fact that in Eiffel, a class is both a type and a module,
whereas in Ada, we have types, subtypes, and packages as separate
concepts.

>> but there are some problems with it. One 
>> problem is that such types are very confusing in array indexes/slices (do 
>> the values all participate or just the ones that pass the predicate?) 

I consider such problems to be side issues.  They are solvable,
I think.

> One way to solve^H^H^H^H^H^H circumvent this problem would be to prohibit 
> that kind of subtyping for discrete types. In Dmity's example above, 
> Window_Type is likely to be a record (probably a tagged one). If it is not 
> a discrete type, you can't use it as an array index. :-)

I don't like that "solution" (or "circumvention", if you prefer).  ;-)
I think one important goal is to be able to have arbitrary
subtypes of an enumeration type, in addition to just subranges.
I don't much care about arrays indexed by such subtypes.

    subtype Vowel is Character
        with Predicate => To_Lower(Vowel) in ('a', 'e', 'i', 'o', 'u');

or something like that.  Here, Vowel refers to the current instance
of subtype Vowel.  I don't particularly want an array indexed by
Vowel, but I might want to say:

    case C is
        when Vowel => ...;
        when Consonant => ...;
        when Punctuation => ...;
        ...

or:

    procedure P(X : Vowel);

or:

    if X in Vowel ...

- Bob