[swift-evolution] [Pitch] Remove destructive consumption from Sequence

[Anton Zhilin]

Dave Abrahams via swift-evolution <swift-evolution at ...> writes:

> I should be clear up-front about the main problem I'm trying to solve:
> 
> Today, people see a beautiful, simple protocol (Sequence) to which 
many
> things conform. They don't recognize that there's a semantic 
restriction
> (assume you can make only a single-pass!) on it, so they write 
libraries
> of functions that may make multiple passes over Sequences.  They test
> their libraries with the most commonly-available Sequences, e.g. 
Arrays
> and Ranges, which happen to be multi-pass.  Their tests pass!  But 
their
> constraints are wrong, their whole model of how to write generic code
> over sequences is wrong, and some of their code is wrong.

This is a mistake that I see more often than I want to: designers of 
development tools underestimate abilities of developers, and end up 
restricting many of them.

My point is that we should impose minimal requirements on custom data 
types. Quoting yourself:

> In this case, protocols used to interface with the language
> at the lowest levels may be purely about syntax.

So there should be a protocol for building into for-in loops that 
requires only that the sequence should be iterable once. Currently, we 
have Sequence for that. If we remove destructive consumption from 
Sequence, then there should be a super-protocol SinglePassSequence or 
Forinable (excuse the pun).

But even if we do this, I still don't understand the intentions. If a 
Sequence is multi-pass, then we can satisfy requirements of Collection, 
as already noted in the discussion. Why should we keep Sequence at all, 
then? Then let's remove it and rename SinglePassSequence to Sequence???