Re: Pb with use of redefined "=" operator

[Mats Weber <Mats.Weber@elca-matrix.ch>]

Matthew Heaney wrote:

> ALWAYS import equality for any non-tagged formal type.  In the data
> structure library I'm building, every private type is imported as
> 
> generic
> 
>    type Item_Type is private;
> 
>    with function "="
>      (L, R : Item_Type)
>      return Boolean is <>;
> ...
> package ACL.Stacks.Adapters is ...

I would advise against this, because predefined equality will reemerge
in other situations, such as this:

    type Pair is 
       record
          X, Y : Item_Type;
       end record;

"=" (Pair, Pair) will use the predefined equality on Item_Type, so that
you have to write a new "=" function for Pair. This makes it very hard
to check that the code is correct (i.e. every record/array declaration
that contains a generic formal type must have its own redefined "=").

My advice is that you must know about the reemergence problem and never
instantiate a generic with a type whose predefined "=" does not work.
You can often do this it using tagged types.