Re: Parameter Modes, In In Out and Out

[dmitry6243@my-deja.com]

In article <93l89r$r9m$1@nnrp1.deja.com>,
  mark_lundquist@my-deja.com wrote:
> In article <93cagm$c1j$1@nnrp1.deja.com>,
>   dmitry6243@my-deja.com wrote:

> > Maybe I am wrong, but it seems that RM does not forbid to keep the
> > type tag separated from the value. Does it?
>
> No, it doesn't; it's just that the tag is not "part of" the value
> (regardless of where it is "kept").
>
> > Anyway the clear distinction of normal types and class-wide types is
> > an excellent (revolutionaly, IMO) concept. It is pity that Ada 95
> > didn't use all advantages of the concept. For instance, to have
> > multiple dispatch, or even, all types "tagged"!
>
> Interesting idea, but I don't know that I agree it's "a pity" that Ada
> didn't do these things. Ada is a practical language.  Making scalars,
> etc. carry a tag around wouldn't be worth it. It would really foul up
> system-level programming and interfacing to other languages.

No, you need a tag only in a class-wide. 90% of all code does not
involve class-wide values, references or pointers. I do not think that
Integer's + and - should be dispaching (:-)) even if Integer
is "tagged". 'Value and 'Image are much better candidates.

In any case if the tag is kept separately you can interface to all
other languages until you make dispatchig calls. To have it safe, there
should be a way to say which parameter of a primitive operation is
dispatching and which is not. If all parameters are non-dispatching,
you have a normal subroutine suitable to interface even with FORTRAN-IV.

> I don't see a whole lot of value in doing classwide programming on
> integers or arrays anyway (with plenty of opportunity for obscure
> code). Can't you get the same effect by declaring a tagged container,
> then deriving from that?

What about implementation of the attributes like 'Image, 'Read, 'Output
etc. as dispatching subroutines? I made no detailed investigation, but
I think that many currently hard-coded things could be then expressed
in the terms of the language.

--
Regards,
Dmitry Kazakov


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