Re: Ada Distilled by Richard Riehle

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2018-07-28 05:52, Dan'l Miller wrote:
> On Friday, July 27, 2018 at 3:07:21 PM UTC-5, Dmitry A. Kazakov wrote:

>> Set-theoretic operations have only limited interest.
> 
> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.51.200&rep=rep1&type=pdf
> I disagree.  From 2 decades ago, Holzmüller & Plödereder seem to find quite a fascinating use for one set-theoretic operation in particular:  union/conjunction as the basis for openmultimethods dispatched more like PL/I generics (dispatched on subprograms by curated type) than Ada95/C++ single-dispatch OO (i.e., dispatched on types by curated subprogram).

Ad-hoc supertype would do that job. Instead of building a class by 
specifying its members:

    type T is union (T1, T2, T3);

as authors propose, we could simply declare a common ancestor:

    type Semantics_Interface is ...
    procedure Eval (X : in out Semantics_Interface) is abstract;

for all tree types.

Then for each type we create a supertype that also inherits from 
Semantics_Interface. Doing so we will be required to override Eval.

Now all three types are in the same class Semantics_Interface'Class and 
we can dispatch on Eval on class-wide instances from 
Semantics_Interface'Class.

The advantage of this approach is that it is still inheritance, nothing 
new, just Ada OO model. Another important advantage is that members of 
the ad-hoc class need not to be all known in advance. We can add members 
as needed in separate packages.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de