CollectionType+traverse.swift

import PromiseKit

extension CollectionType {
  func traverse<T>(transform: Generator.Element -> Promise<T>) -> Promise<[T]> {
    return Promise { fulfill, reject in
      var accumulator: [T] = []
      var generator = self.generate()
      func recurse() {
        guard let next = generator.next() else {
          fulfill(accumulator)
          return
        }
        transform(next).then { result -> () in
          accumulator.append(result)
          recurse()
        }
      }
      recurse()
    }
  }
}