Re: Run-time accessibility checks

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:k2ftogbqmojg.yrjw3ldbi604.dlg@40tude.net...
...
>> Moreover, elements of many forms of
>> container don't have any order, so you can't do anything useful with the
>> index by itself -- but operations like iteration are still meaningful.
>
> No, if index is unordered and not dense, then iteration is meaningless. It
> is a broken abstraction then. If you want to iterate a container you have
> to order its elements. That is equivalent to existence some ordered dense
> index of.

Not at all. You are saying that "forall" (that is, do something to all 
elements of a container) is not a useful iteration if the elements don't 
have an order. That's pretty hard to believe; indeed, the value of an 
iterator is is specifically because the order of the elements is not 
specified; without that, there is no way to get all of the elements via 
their cursors (or indexes, if you prefer).

> The index can be of the type System.Address, that is no matter.
> But to be able to iterate is a contract on the container. It is IMO a very
> bad idea to assume all containers to fulfill this contract.

I would never want to say *never* (there is always a counter-example), but 
surely all of the classic containers (including a "bag") need iteration. 
It's true that the implementation of a "bag" will necessarily impose an 
order on the elements, but that order is not interesting and shouldn't be 
considered part of the abstraction. But the ability to get all elements of a 
bag for processing is a fundamental operation of the container.

                              Randy.