数组flatMap

gistfile1.txt

https://github.com/apple/swift/blob/master/stdlib/public/core/SequenceAlgorithms.swift.gyb

@_inlineable
public func flatMap<SegmentOfResult : Sequence>(_ transform: (${GElement}) throws -> SegmentOfResult) rethrows -> [SegmentOfResult.${GElement}] {
    var result: [SegmentOfResult.${GElement}] = []
    for element in self {
      result.append(contentsOf: try transform(element))
    }
    return result
}

@_inlineable
public func flatMap<ElementOfResult>(_ transform: (${GElement}) throws -> ElementOfResult?) rethrows -> [ElementOfResult] {
    var result: [ElementOfResult] = []
    for element in self {
      if let newElement = try transform(element) {
        result.append(newElement)
      }
    }
    return result
}