Re: Eiffel and Java + Ada dispatching

[donh@syd.csa.com.au (Don Harrison)]

Robert I. Eachus writes:

:   In Ada, the default for generics is that you get the "predefined"
:operations for the class.  But if you explicitly pass the operation as
:a generic formal, you can override the standard definition.  No
:surprise there either.  

Okay. If it made sense, I think I would prefer to encapsulate behaviour of 
related operations into a type and define descendants covering the variants. 
Then the parent type bundled with its operations can be used as a formal 
generic parameter. That way, you simplify the generic interface. Don't know 
whether this fits at all.

:The suprise in the construct I demonstrated is
:that it is possible to define generics that are sensitive to the
:environment where they are instantiated.
:
:   And yes, it is a very useful feature, and should be used very
:carefully.  I have written several Ada 83 libraries where generics
:have use this feature extensively, and have as many as thirty formal
:parameters.  Since the packages are intended to be useful, they also
:are designed so that most of the generic formals have default values,
:so the instantiations only need a few lines.

If you were writing this now in Ada95, would you be able to reduce the 
complexity by using OO stuff?

:   But it is a high (and almost lanaguage independent) talent to
:observe the principle of least surprise.

Indeed. :)

:  > If the non-overloading versions are exponentially larger, maybe
:  > the originals were a bit dodgy anyway. :)

In saying this, I didn't realise you were talking about Ada83. It makes more
sense in that context.

:   No, but the most extreme case I know of was real portmanteau code.
:Generic instantiations with other generic instantiations as
:parameters, about four levels deep.  

Nasty stuff!

:But any other way of writing the
:code would have required millions of lines of code.  The several
:thousand generic instantiations weren't easy to debug and test, but
:much much better than any million line monstrosity would have been.
:Just don't use the powerful features for the easy jobs, save them for
:when and where they are needed.

I'll interpret 'powerful' to mean 'esoteric'. :)

:  > IMHO, overloading should have no semantic impact. It should merely be 
:  > 'syntactic sugar' as Jean-Marc suggested.
: 
:   Nice dream, but...the fact that horses are a good way to commute
:shouldn't lead to the outlawing of jet planes for intercontinental
:travel.  However, you do need to make sure that the airplane pilot
:knows how to fly. ;-)

Yes, and thankfully they make planes *easier* to fly these days. :)

Jon should know something about that.


Don.
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Don Harrison             donh@syd.csa.com.au