Re: Proposed change to BC iterator parameters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"amado.alves" <amado.alves@netcabo.pt> writes:

> <<...
>    generic
>       type Param_Type is private;
>       with procedure Apply (Elem : in Item;
>                             Param : in Param_Type;
>                             OK : out Boolean);
>    procedure Visit_With_In_Param (Using : in out Iterator'Class;
>                                   Param : in Param_Type);
> >>
> 
> I've never found a situation where the following much simpler signature wouldn't suffice:
> 
>    generic
>       with procedure Apply (Elem : in Item);
>    procedure Visit_With_In_Param (Using : in out Iterator'Class);
> 
> with premature termination of the iteration done by raising inside
> Apply an exception which Visit_With_In_Param propagates by
> definition. 

Yeah, that works.  It's not very efficient (in most implementations).
If you want to stop the loop as soon as you found the thing you're
looking for, it's quite likely that the exception-handling overhead is
more than the cost of letting the loop keep going to the end.

Ada really doesn't have very good support for iterators.  This is one
example.  The iterator abstraction should not have to worry about
premature exits -- the loop code should just jump out of the loop.
A goto statement would be better than an exception here, because you're
going to a specific place (as opposed to raising an exception that could
be handled who-knows-where).  You can do that in Pascal, but Ada doesn't
allow gotos out of procedures (for good reason -- consider a procedure
called by a nested task).

(Goto haters should consider that exceptions are just like gotos, except
you don't know statically where they're going to -- i.e., exceptions are
*worse* than gotos, from a structured programming point of view.)

I don't understand the point of Param_Type and Param above.  If you want
to pass extra information to Apply, nest the instantiation in a place
where it can see that data.

- Bob