[swift-evolution] [RFC] New collections model: collections advance indices

Dave Abrahams dabrahams at apple.com
Tue Mar 8 11:21:19 CST 2016

on Tue Mar 08 2016, Антон Жилин <swift-evolution at swift.org> wrote:

> In the RFC, I see that indices only requires `next`. 

In the RFC, there's no Index protocol at all;  Indices
are required to be Comparable.

> I like this scheme a lot, it's very altruistic, like in C++. Get,
> increment, end check - nothing more is required to work with
> iterators.

FWIW, building a correct C++ iterator is so boilerplate-heavy and
occasionally subtle that people use libraries such as

> I like extracting everything optional to Collection hierarchy, and don't
> like adding Equatable, Comparable or Hashable constraints on
> iterators. 

You can barely write any interesting generic algorithms at all without
the moral equivalent of Equatable indices.  

> If you need Hashable in a specific scope, declare it as a generic
> constraint.  I didnt really read the whole RFC, so I could have
> misunderstood something.

I don't support making indices Hashable at this time; IMO that's part of
a much larger discussion that we need to have about regular types and
implicitly-generated conformances.  Comparable is arguable either way,
but we are proposing to require it in part to be able to test the
semantics of a valid Range.


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