Re: Eiffel and Java + Ada dispatching

[jhd@herold.franken.de (Joachim Durchholz)]

nebbe@iam.unibe.ch wrote 05.11.96:

> By the way, consider what Eiffel does. It determines which routine is to be
> called based on the name of the routine and the type of one of the
> parameters. The only difference with Ada is that Ada uses all the
> parameters.

I don't think so. Overloading in Ada determines which routine to call _at_  
_compile_ _time_. This is very different from Eiffel, which uses the type  
of one of the parameters to determine which routine to call, _at_ _run_  
_time_ (this is called dynamic binding).

> There is no semantic difference between determining statically
> or dynamically which method is invoked. When Eiffel compilers optimize away
> dispatching all they are really doing is static overload resolution.

In a sense, you are right. In the absence of polymorphism, dynamic binding  
degenerates to overloading. (No downplay on overloading intended - I found  
it quite useful, though it also makes a great pistol if you don't control  
the semantics very carefully. Dynamic binding is an even better pistol,  
but luckily the programming-by-contract rules provide an effective safety  
catch.)

However, the central difference is that dynamic binding is always there in  
Eiffel. You don't have to rewrite Eiffel code to make overloading work,  
you just don't use polymorphism. You'd have to rewrite Ada code to go from  
overloading to dynamic binding.

The situation is a bit similar to multiplication optimization. Compilers  
can optimize multiplications with a power of two by issuing left-shift  
instructions instead of multiplications. But programming languages don't  
require the programmer to write all multiplications in terms of left-shift  
operators (even C compilers don't enforce this ;) ), and programmers  
usually don't think about the difference.
Overloading on the first routine parameter is a similar case for an Eiffel  
programmer; there is no reason why he should consider this special case as  
anything peculiar.

Regards,
-Joachim

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