[swift-evolution] [Review] Add a Lazy flatMap for Sequences of Optionals

[Donnacha Oisín Kidney]

Now that I think about it, of course CollectionOfZeroOrOne would (or Optional) have a random access index. I’ll update the proposal to include that.

I really like the CollectionOfZeroOrOne solution. The semantics and types certainly make more sense. I’m not sure that it reduces the amount of code, though: the three extensions are still needed, plus an extra struct. (I suppose that’s not counting the extra version of LazyFilterCollection. Am I right in saying that that is intended to be added?)

Here’s what I have for the collection (I tried to mimic the standard library’s CollectionOfOne):

public struct CollectionOfZeroOrOne<Element> : CollectionType {

  public typealias Index = Bit
  
  public init(_ element: Element?) {
    self.element = element
  }
  
  public var startIndex: Index {
    return .Zero
  }
  
  public var endIndex: Index {
    switch element {
    case .Some: return .One
    case .None: return .Zero
    }
  }
  
  public func generate() -> GeneratorOfOne<Element> {
    return GeneratorOfOne(element)
  }
  
  public subscript(position: Index) -> Element {
    if case .Zero = position, let result = element {
      return result
    } else {
      fatalError("Index out of range")
    }
  }
  
  let element: Element?
}

Does that seem reasonable?

> On 16 Dec 2015, at 02:29, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> First, I’d like to thank Oisin <https://github.com/oisdk> for his proposal.  It’s great to see people filling in gaps.
> 
>> 	* What is your evaluation of the proposal?
> 
> The basic idea is solid and obviously appropriate.  I have some quibbles with the proposed solution.
> 
>> 	* Is the problem being addressed significant enough to warrant a change to Swift?
> 
> Yes; it’s a non-uniformity, nonuniformities create complexity for users.
> 
>> 	* Does this proposal fit well with the feel and direction of Swift?
> 
> Yes.
> 
>> 	* How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
> 
> A quick reading, but I’m deeply familiar with the domain, having written the lazy facilities we have in the stdlib.
> 
> I don’t see why the proposal states, “Optional probably wouldn't have a BidirectionalIndexType“.  It seems to me that as a collection, an optional’s index should be random access, just like CollectionOfOne’s.
> 
> I’d like to suggest a different implementation approach that not only handles the bidirectional issue but also probably reduces the amount of code involved: create a generic CollectionOfZeroOrOne<T> that wraps a T?, and implement the x.flatmap(f) where f returns an optional as x.flatmap { CollectionOfZeroOrOne(f($0)) }
> 
> What do you think?
> 
> -Dave
> 
> 
> 
> 
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