flatMapAll.swift

extension Sequence {
  /// Returns an array with the transformed elements of the sequence if all
  /// the transformations succeed.
  func flatMapAll<T>(_ transform: (Element) -> T?) -> [T]? {
    var result: [T] = []
    result.reserveCapacity(underestimatedCount)
    for element in self {
      if let transformed = transform(element) {
        result.append(transformed)
      } else {
        return nil
      }
    }
    return result
  }
}

let valid = ["1", "2", "3", "4"]
print(valid.flatMapAll(Int.init))
// Optional([1, 2, 3, 4])

let invalid = ["1", "2", "blah", "4"]
print(invalid.flatMapAll(Int.init))
// nil