Re: Multiple dispatch (was Re: Parameter Modes, In In Out and Out

[dmitry6243@my-deja.com]

In article <95dm8r$a9a$1@nnrp1.deja.com>,
  mark_lundquist@my-deja.com wrote:

> If they expected triple dispatch, then they don't know Ada :-)

Well, in fact Ada does have "MD", defined as follows. If all tags are
same, then dispatch happens as it is expected. If the tags differ the
call is "dispatched" to a "method" that raises an exception. (Quotation
marks are placed for purists (:-)). Run-time exception is an obvious
problem and I can easily imagine applications where such behaviour
would be unacceptable.

> Maybe they really expected covariance, in which case they should have
> used a generic instead.
>
> Another possibility is that the inputs to Compute should be of type
> FuzzyNumber, not FuzzyNumber'Class.  Users of Compute would upcast as
> necessary.  (However, they somehow have to have some reason to believe
> that upcasting yields a meaningful result, which is hard to know
> without seeing into the implementations of the abstractions.  This is
> a general difficulty with programming by extension).

That's another question. No programming by extension was intended. It
was rather an attempt to have different respresentations for same
thing (FuzzyNumber).

> I'm not a computer scientist, and I haven't ever studied multiple
> dispatch, so maybe this is a naive question... but don't you now have
> a problem with cross-coupled sibling dependencies?  Suppose I create
> another type Wild_And_Wooly_Number, derived also from FuzzyNumber?
> How do you fill in your table then?

It is not a naive question. We could say (1) Wild_And_Wooly_Number
knows nothing about SpecialFuzzyNumber. Then a definition of + for any
mixture of SpecialFuzzyNumber and Wild_And_Wooly_Number is not a method
=> not an overriding => either an overloading or illegal. Alternatively
(2) we "just" have to define (no matter implicit or explicit) all
possible combinations involving Wild_And_Wooly_Number and any of
existing successors of FuzzyNumber. If Wild_And_Wooly_Number does not
override +, then the compiler does it implicitly (by taking an
appropritate + where each appearance of Wild_And_Wooly_Number is
replaced by FuzzyNumber). If a programmer overrides at least one, then
there are two possibilities: either to use the default for others (=
unexpected behaviour) or to force the programmer to override all of them
too (= combinatoric explosion).

As I already said in my previous postings, I do not know the answer. I
even do not know whether the answer exists (:-()

--
Regards,
Dmitry Kazakov


Sent via Deja.com
http://www.deja.com/