[swift-evolution] [Accepted with modifications] SE-0045: Add scan, prefix(while:), drop(while:), and unfold to the stdlib

[Patrick Smith]

Or new AnySequence constructors? So the sequence can be generated more than
once.

  

let a = AnySequence(from: 1){ $0 * 2 }.prefix(10)  
print(Array(a)) // [1, 2, 4, 8, 16, 32, 64, 128, 256, 512]  
  
let b = AnySequence(from: 10){ $0 == 0 ? nil : $0 - 1 }  
print(Array(b)) // [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]  

  

  

Code as implemented in Swift 2.2:

  

extension AnySequence {  
 init(from: Element, applying: (Element) -> Element) {  
   self.init({ () -> AnyGenerator<Element> in  
     var current: Element?  
     return AnyGenerator{  
       current = current.map(applying) ?? from  
       return current  
     }  
   })  
 }  
  
 init(from: Element, applying: (Element) -> Element?) {  
   self.init({ () -> AnyGenerator<Element> in  
     var current: Element?  
     return AnyGenerator{  
       current = current.map(applying) ?? from  
       return current  
     }  
   })  
 }  
}  

  

  
**Patrick Smith**  

On May 6 2016, at 7:27 am, Brent Royal-Gordon via swift-evolution <swift-
evolution at swift.org> wrote:  

> > One idea that came out of the core team discussion was something like:  
>  
> sequence(from: 0) { $0 += 42 }  
>  
> Since it returns a sequence.

>

> It definitely occurred to me that this was kind of just a way to construct a
generic iterator. Maybe a new AnyIterator (I believe there is such a thing)
constructor?

>

> \--  
Brent Royal-Gordon  
Sent from my iPhone  
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