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

[Dave Abrahams]

> On Dec 15, 2015, at 7:21 PM, Rob Mayoff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Dmitri wrote:
> What's the advantage?  Why would we want to have a type that is isomorphic to Optional, except that conforms to CollectionType?
> 
> My question (apologies if it has been answered already) is: Why isn't Optional a CollectionType, like it is in Scala?

I don't remember the rationale, but it used to be that way <https://github.com/apple/swift/commit/fad874708e05cff56aec5587a4b0f49cdadc6d11> and during the run-up to the Swift 1 release IIRC several members of the Swift team objected to it.  That's the answer to Dmitri's question: I have considered that to be off the table, but we could revisit it.  

One cute effect, which might be too cute for some, is that

  if let x = y {  ... } 

becomes equivalent to

  for x in y { ... }

when y is an optional.

-Dave



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