Re: Generic Package

[Simon Wright <simon.j.wright@mac.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Mon, 30 Apr 2007 16:48:53 +0200, Georg Bauhaus wrote:

>> "The foreach procedure operates as if the following body
>> were written
>> 
>>     procedure foreach(S: Set) is
>>        C: Cursor;
>>     begin
>>        C := first(S);
>
> You forgot that first is not defined on Set, which is not
> well-ordered and thus does not have visible implementations of
> first, last, next, prev etc in its interface. It has Length, =, /=,
> :=, and, or, xor, /, Empty, in, not in. [*]

First is not *publicly* defined on Set but this is *implementation*,
for Pete's sake! It's likely to be hard to implement iteration/foreach
efficiently without private knowledge .. but you could, for example,
copy the Set and then pick a random member, remove it from the copy,
and process it, until the copy was empty.