Re: Generic Package

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

On Fri, 27 Apr 2007 23:44:04 +0200, Georg Bauhaus wrote:

> On Fri, 2007-04-27 at 16:16 +0200, Dmitry A. Kazakov wrote:
>> On Fri, 27 Apr 2007 16:10:47 +0200, Georg Bauhaus wrote:
>> 
>>> On Fri, 2007-04-27 at 14:06 +0200, Dmitry A. Kazakov wrote:
>>> 
>>>> It does. Semantically, table /= a snapshot of. That is.
>>> 
>>> For Iterate, a snapshot is a table, a collection of tuples.
>> 
>> Well, for a hammer everything is a collection of nails.
> 
> Please. From a user's perspective Iterate is the important
> interface.

Yes, but it is not the interface of a set of tuples. It is an interface of
some ordered container.

> When I call Iterate to apply an operation to
> each tuple of a table, an order is established, even though
> it does not matter.

This is too swampy. There is a sufficient difference between foreach
closures and interators. You can have foreach on an unordered set, but you
cannot have iterators. The difference is in who has the control over the
enumeration process. Iterators are more binding for the container and less
for the user of.

Consider a library. Say you wished to read all books there. You come to the
librarian and order a book. Then you read that book and return. Now, you
can ask him: give me another book. But you cannot require him to give you a
book that you haven't yet read. That is your business, he would answer.

The librarian interface is of an unordered set (no iterators). Yet you
could give him a set of post-it notes with numbers on them and ask him to
put one on each of the books. (Later you probably could ask him for a book
with the specific number on it). He could still refuse but then you would
come back with a flame thrower and burn the library and the damned books
down. Both were examples of foreach.

>  I really don't understand why Iterate should not be able
> to produce a positional number for each tuple it produces.

Because it is not how relational algebra was specified. If you are
objecting to its idea, well, I am with you, and the most of DB vendors are
as well, as they all provide row IDs to ease our life.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de