filter.swift

public struct FilterGenerator<G : GeneratorType> : GeneratorType {
  private var generator: G
  private let predicate: G.Element -> Bool
  public mutating func next() -> G.Element? {
    while let next = generator.next() { if predicate(next) { return next } }
    return nil
  }
}

public struct FilterSequence<S : SequenceType> : SequenceType {
  private let sequence : S
  private let predicate: S.Generator.Element -> Bool
  public func generate() -> FilterGenerator<S.Generator> {
    return FilterGenerator(generator: sequence.generate(), predicate: predicate)
  }
}

extension LazySequence {
  public func filter(includeElement: Generator.Element -> Bool) -> FilterSequence<LazySequence> {
    return FilterSequence(sequence: self, predicate: includeElement)
  }
}