fib.swift

var (a, b) = (1, 0)

var fibs = lazy(
  
  GeneratorOf<Int> {
    
    (b, a) = (a, b + a)
    
    return b
    
  }
)

extension LazySequence {
  
  func takeWhile(condition: S.Generator.Element -> Bool)
    -> LazySequence<GeneratorOf<S.Generator.Element>> {
      
    var gen = self.generate()
    
    return lazy( GeneratorOf{ gen.next().flatMap{ condition($0) ? $0 : nil }})
      
  }
  
  func reduce<U>(initial: U, combine: (U, S.Generator.Element) -> U) -> U {
    
    var accu = initial
    
    for element in self { accu = combine(accu, element) }
    
    return accu
    
  }
  
}

fibs
  .filter{$0 % 2 == 0}
  .takeWhile{$0 < 100}
  .reduce(0, combine: +)