[swift-evolution] Proposal: Add SequenceType.first

[Dave Abrahams]

> On Jan 2, 2016, at 11:26 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On Sat, Jan 2, 2016, at 11:17 PM, Brent Royal-Gordon wrote:
>>> `buffered` is no more problematic than `lazy` is. In fact, calling `buffered` actually doesn't have any side-effects at all (it can avoid fetching the first element until you call `first` on the result of `buffered`).
>> 
>> If `seq` is a single-pass sequence, then `seq.buffered.first` will consume an element from `seq`, even though you only accessed two properties. That's why I call it problematic.
>> 
>> (If calling `buffered` somehow rendered the original sequence unusable—for instance, if we had some way to express that the `BufferedSequence` takes unique ownership of its base—this wouldn't bother me as much.)
> 
> If `sequence` is a single-pass sequence, wrapping it in any other sequence type and then doing anything with that other sequence type makes the original sequence unusable (or rather, you can still use it but the elements yielded from any further access to the original sequence can be completely arbitrary).
> 
> And for the record we already have precedent for the specific case of `seq.prop1.prop2` destructively consuming the original sequence: `seq.lazy.array`.

Yes, and there are arguments for dropping “.array” as a property.  The convention is that “conversions” (ill-defined, I know) use constructor syntax, and we are currently heading towards the elimination of "convenience” interfaces that duplicate functionality, so we might end up with Array(seq).  

All that said, single-pass Sequences are just weird in that they get mutated without calling any mutating methods on them; you mutate them by calling a mutating method on a separate generator instance.  In other words, they fundamentally have reference semantics.  There may be some better way to address this whole area, but we’ll have to go much deeper than merely poking at the question of a  `.first` property. 

-Dave