Re: Is ADA as good for graphics programming as C? (WAS: Re: Avoiding the second historic mistake)

[mheaney@ni.net (Matthew Heaney)]

In article <33C4AF0E.339D@calfp.co.uk>, Nick Leaton <nickle@calfp.co.uk> wrote:

>> Different languages have made different choices with respect to whether all
>> types should be part of a common hierarchy.  That Eiffel did so, and Ada
>> did not, should not be construed as better, just different.
>
>I'm not sure about this. If you don't have a common hierarchy, then that
>precludes you from using a common root. Now this is useful when we look
>at container classes. It is particularly useful when you can constrain
>the type that is store in a container to a particular type. 

I think there is confusion here between what one does versus how one does it.

In Ada, you can constrain the type you use to parameterize a container
class, it's just that the mechanism for doing so is different from Eiffel.

For example, I can parameterize a type with any assignable type:

generic
   type T is private;
package Containers_G is

   type Root_Container is abstract tagged private;
...

I can constrain the generic formal type so that it only allows integers:

generic
   type T is range <>;
package Containers_G is ...;

I can constrain the type to only integer types that derive from integer
type T (example *):

generic
   type NT is new T;
package Containers_G is ...;

I can constrain the type to any type with an addition operator:

generic
   type T is private;
   with function "+" (L, R : T) return T is <>;
package Containers_G is ...;

I can constrain the type so that it only takes tagged types (those that
have type extension):

generic
   type T is tagged private;
package Containers_G is ...;

I can constrain the type so that it only takes tagged types that derive
from a specific tagged type T (example **):

generic 
   type NT is new T with private;
package Containers_G is ...;

Examples (*) and (**) are examples of "constrained genericity," which is
exactly what you said would be really nice to have.  So you have it an
Eiffel AND Ada.

So what's the problem?   In Ada, you can constrain the type used to
parameterize another, without all types necessarily being related via a
hierarchy.  That Eiffel does it differently from Ada should be construed
only to mean that Eiffel does it differently from Ada, not better.

In Ada, you can even do mixin inheritance without using multiple
inheritance.  Imagine that!

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Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
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