[swift-evolution] Proposal: Add SequenceType.first

[Kevin Ballard]

On Thu, Dec 31, 2015, at 12:36 AM, Dave Abrahams wrote:
>> On Dec 30, 2015, at 3:57 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
>>
>> It's sometimes useful to get the first element of a sequence. To that
>> end I'd like to propose
>>
>> extensionSequenceType { /// Returns the first element of
>> `self`, or `nil` if `self` is empty. /// - Complexity: O(1) var
>> first: Self.Generator.Element? { var gen = generate() return
>> gen.next()    } }
>>
>> I think it makes sense to add this property to the definition of
>> SequenceType as well, so various sequences can override it to avoid
>> constructing a generator.
>>
>> With this added to SequenceType, we can remove it from
>> CollectionType, as the behavior will be the same.
>
> Property accesses should not mutate the receiver, and because of how
> Sequences work, inspecting first may consume the first element.

Fair enough. Swift does support `mutating get`, but it looks like the
stdlib doesn't actually use this anywhere (at least, in the public API).

Still, it's a shame that there's no way to do this generically for
sequences. It's something I need upon occasion. I wish I could actually
say something like `seq.generate().next()`, but you can't call mutating
functions on temporaries.

I'd be tempted to say we should have a `func first()` method, except
then CollectionTypes would have both a property and a method named
`first` and that would just be confusing.

> I suggest you consider adding a BufferedSequence<Base: SequenceType>
> that has a stable first property (at least until it is iterated)

Good idea, though I'd probably call it PeekSequence because it would
only buffer a single element (and BufferedSequence sounds like it's got
an arbitrary-sized buffer). Perhaps more useful would be the associated
PeekGenerator, because peek() is a useful thing to have when writing
custom generator-using code.

I'll write up a more detailed email with a proposed design in a minute.

> Another related adapter I’d like to add is a model of
> CollectionType that is backed by a sequence and lazily populated in
> fixed-sized chunks.

Also a good idea. Although really it could just be backed by a
ContiguousArray, those things already grow in chunks. I'll write up a
design for that too.

-Kevin Ballard
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