Re: Private discrete type as index

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

AG wrote:

> package test is
> 
> type X is new Integer; -- What I'd like here is "limited private" instead
> of "new integer"
> 
> type Test is array(X) of Boolean;
> 
> function "+"(Left: X) return X; -- Unary increment (however that may be
> implemented)
> 
> function "+"(Left:X; N: Integer) return X; -- Shift by N positions (does
> not equal N times other "+")
> 
> private
> 
> -- Defiinition of "X" here whatever it may be
> 
> end test;
> 
> The code above compiles fine. Replacing "new Integer" with "limited
> private" does not.
> 
> But, if it did, it would allow the user of the package to do things like
> 10th position from
> 
> wherever the index is now and it would be the responsibility of the
> implementation
> 
> to know what/where it is. At the same time, the client package would have
> no clue
> 
> as to what the type is or be able to operate on it directly (only through
> any functions
> 
> provided by the package which was the whole point).

Ada does not have user index types other than publicly derived from discrete 
types. "limited private" is not such a type. You could make a sort of 
private type out of a discrete type by disallowing some of its predefined 
operations (making them abstract), but you are unable to make it limited, 
because ":=" is not overridable. Neither can you expose a private type as 
an array. Alas, ADT is far incomplete in Ada.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de