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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Your wish list for Ada 202X
Date: Tue, 15 Apr 2014 16:20:00 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2014-04-15T16:20:00-05:00
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"Natasha Kerensikova" <lithiumcat@instinctive.eu> wrote in message 
news:slrnlkpnfe.i0l.lithiumcat@nat.rebma.instinctive.eu...
...
> let's consider an abstract type T, with an abstract operation AO and a
> huge part of the algorithm implemented in primitive operations for T,
> making use of AO. Concrete descendants of T are supposed to implement
> AO, and inherit the rest without overriding.
>
> Then in T subprograms I end up with things that look like:
>   AO (T'Class (Self), Other, Arguments);
>
> I've always thought of this as "re-disatching". Is it?

I think it is as well.

> Is it somehow more acceptable? (maybe it doesn't make any sense to
> consider it as some "predicate with the same object viewed as T")

Perhaps. The only cases where I've intentionally used re-dispatching look 
like this (one example: action routines in Claw, which are of course 
implemented by each extension -- but the bulk of the work is shared by all 
implementations). Such a case seems somehow different, but I don't think I 
can explain why.

> Are there better language facilities to use in such situations?
> (I used to use generics for that, and still do when there is absolutely
> no plan for any child package)

I don't know of any. One could use access-to-subprograms for such usage, but 
I don't see any big advantage to doing that instead -- there seems to be 
more chance of getting something wrong that way. (Failure to register, 
registering the wrong routine, etc. can't happen with the overridding 
routine solution.)

I suspect that the reason re-dispatching is allowed is because there are 
some cases like this where nothing else would seem to work as well. But 
perhaps there is a better solution, we just haven't found it yet.

                                  Randy.