Re: Two ideas for the next Ada Standard

[dewar@cs.nyu.edu (Robert Dewar)]

Richard said

"That's one of the things I like about the old Russell type system:
you can derive a new type by weakening.  I can say "T is integer
except that it doesn't have these operations."

>This includes add, multiply, subtract, 'Image, and all the
>other built in stuff,

That is indeed a weakness in Ada.  It should be possible to exclude
such things.  It is presently possible to do that by exporting a
private type and replicating all and only the things I _do_ want to
offer, so I don't call it a defect."



Richard, you can't quite do 100% of what you want, since you cannot exclude
attributes like Image, but you are obviously missing an important capability
in Aa 95.

	type x is range 1..10;
        function "+" (a,b : x) return x is abstract;

causes the addition operator to be unavailable for this type.
This is how weakening is done in Ada 95.

In general it is a good idea to turn any statement of the form "You cannot
do xxx in Ada 95" to a question "How do you do xxx in Ada 95?" unless you
are very sure of your facts. That way you avoid stating misconceptions as
facts on the newsgroups.

It's an interesting point that it is a pity that there is no way to suppress
attributes. That's particularly true in the floating-point case, where there
are huge numbers of attributes, or in the enumeration type case, where it
would be nice to suppress Pos, Succ, and Pred.

The other operation that you cannot suppress is the ability to take
subranges, which, particularly in the enumeration type case, can be
unfortunate. The reason is that it allows clients to depend on the
order of items in the enumeration definition, which is really the
business of the defining package. You can supppress the comparison
operations, but not the IN and NOT IN operations on user defined
subtypes, which are essentially semanticaly equivalent to the
comparison operations.