Re: How to test object hierarchy

["Robert I. Eachus" <rieachus@comcast.net>]

Dmitry A. Kazakov wrote:

> This is an argument in favor of implementing MD in Ada, I suppose (:-))

No, it is an argument that current Ada rules are just fine.  You can do 
multiple dispatching in Ada, and the sort of casual dispatching where 
the author of the code doesn't even know that multiple dispatch is 
involves works just fine.  For example, instantiate a generic with a 
classwide type or two as an actual type parameter.  The resulting 
executable may have nested dispatching, but there is no real need to 
anticipate that case when writing the generic.

As for cases where you really NEED to understand the code in multiple 
dispatching terms, these are no harder to program in Ada than in other 
languages--and no easier.  And they really are that hard to wrap the 
mind around.  There are cases which are worth the effort, but precious few.

Another more prosaic example would be a general graphic file format 
converter.  I can design such a tool as a combination of a reader and a 
writer with a standard intermediate representation:

...
declare
   Temp: Intermediate_Format := Read(Input_File);
begin
   Write(Temp, Output_File);
end;

Of course I can rewrite this as:

Write(Read(Input_File), Output_File);

Later I might want to add a graphics format that doesn't work well with 
the "standard" intermediate format.  To do this, I can restructure the 
existing code with a multiple dispatch format:

procedure Process(Input: in Graphics_Format; out Graphics_Format'Class);
   or
function Process(Input: Graphics_Format) return Graphics_Format'Class;

Now I can provide a body which normally uses the old approach with 
Intermediate_Format, but first checks for the special cases and handles 
them with other routines.

THAT is when it starts to get difficult to use multiple dispatch.  It is 
not the process of doing what I call 'casual' multiple dispatch in Ada, 
that is easy.  The problem is going through on an N**2 case by case 
basis (or N**3 or N*M or whatever) and insuring that the approach that 
is dynamically selected for that particular case is, in some sense, the 
right one.  And when the set of possibilities grows, adding one new 
format means considering 2N-1 new cases.

-- 
                                           Robert I. Eachus

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