Re: Re-exporting primitive operations of a private type (like "-", and 'min)

["Jeffrey R. Carter" <spam@spam.com>]

Anonymous Coward wrote:

> That would probably work as long as the definition of Subtract does a
> type conversion explicitly on the Left and Right to the parent types.
> But what are the advantages?  Why would I define a Subtract operation
> and use a renames clause, when I can simply define a "-" operation
> directly?  

No. Subtract is defined for the parent type as a renaming of "-". Thus, calling 
Subtract is the same as calling "-".

The private type derives from the parent type after the declaration of Subtract, 
so it also has Subtract defined as a renaming of the predefined "-". It also has 
a redefined "-" that must be explicitly defined. The redefined "-" is then 
defined as a renaming of Subtract, so calling the redefined "-" is the same as 
calling Subtract, and calling Subtract is the same as calling the predefined "-".

That's kind of a mouthful, but it means the explicitly defined "-" for the 
private type is actually the predefined "-".

The advantage of this approach is that "-" remains the predefined operation for 
the type. It may also be a little shorter and easier to get right.

> The "-" operation must still be replaced, and it still has to have a
> declaration in the public part.  So in your case above, there would
> have to be a 
> 
>   function "-" (Left, Right : in Private_Distance_Type)
>     return Private_Distance_Type; 
> 
> in the public part.

Certainly. That's the only reason you'd go through all this.

-- 
Jeff Carter
"In the frozen land of Nador they were forced to
eat Robin's minstrels, and there was much rejoicing."
Monty Python & the Holy Grail
70