'Array<Element>' 不能转换为 'Array<Int>'

我可以创建一个迭代器来创建前缀：

extension Array where Element == Int {
  func prefixesInt() -> AnyIterator<[Element]> {
    var length = 0
    return AnyIterator {
      guard length < self.count else { return nil }
      length += 1
      return Array(self.prefix(length))
    }
  }
}

for prefix in [1,10,5].prefixesInt() {
  print(prefix)  // Prints: [1]  then  [1, 10]  then [1, 10, 5]
}

现在我想通过以下方式与其他类型实现相同的目标：

1) 将类型编码为 Int

2）使用相同的前缀函数

3）解码回原始类型

[我知道可能有其他方法可以实现这一点，但出于各种原因，我想遵循这条路线，也想了解更多。]

所以我需要：

extension Array where Element: Equatable {
  func encode() -> [Int] {
    return map { self.firstIndex(of: $0)! }
  }
}
print(["A", "B", "A", "C"].encode())  // Prints: [0, 1, 0, 3]

extension Array where Element == Int {
  func decode<Output>(_ original: [Output]) -> [Output] {
    return map { original[$0] }
  }
}
print([2,3,0].decode(["P", "Q", "R", "S"]))  // Prints: ["R", "S", "P"]

我现在可以做我想做的事：

extension Array where Element: Equatable {
  func prefixes() -> LazyMapSequence<AnyIterator<[Int]>, [Element]> {
    return encode().prefixesInt().lazy.map { $0.decode(self) }
  }
}

for prefix in ["H","A","T"].prefixes() {
  print(prefix)
}

我现在想更进一步，将转换（在本例中为前缀）转换为参数，我的尝试是：

extension Array where Element: Equatable {
  func encodeTransformDecode(transform: ([Int]) -> AnyIterator<[Int]> ) -> LazyMapSequence<AnyIterator<[Int]>, [Element]> {
    return transform(encode()).lazy.map { $0.decode(self) }
  }
}

编译得很好但是当我尝试时：

extension Array where Element: Equatable {
  func prefixes2() -> LazyMapSequence<AnyIterator<[Element]>, [Element]> {
    return encodeTransformDecode(transform: prefixesInt) //ERROR: 'Array<Element>' is not convertible to 'Array<Int>'
  }
}

for prefix in ["A","B","C"].prefixes2() {
  print(prefix)
}

然后我得到指示的错误

我被困住了。任何帮助表示赞赏。

全码：

print("========== prefixesInt")
extension Array where Element == Int {
  func prefixesInt() -> AnyIterator<[Element]> {
    var length = 0
    return AnyIterator {
      guard length < self.count else { return nil }
      length += 1
      return Array(self.prefix(length))
    }
  }
}

for prefix in [1,10,5].prefixesInt() {
  print(prefix)  // Prints: [1]  then  [1, 10]  then [1, 10, 5]
}

print("============ encode")
extension Array where Element: Equatable {
  func encode() -> [Int] {
    return map { self.firstIndex(of: $0)! }
  }
}
print(["A", "B", "A", "C"].encode())  // Prints: [0, 1, 0, 3]

print("============ decode")
extension Array where Element == Int {
  func decode<Output>(_ original: [Output]) -> [Output] {
    return map { original[$0] }
  }
}
print([2,3,0].decode(["P", "Q", "R", "S"]))  // Prints: ["R", "S", "P"]

print("============ prefixes")
extension Array where Element: Equatable {
  func prefixes() -> LazyMapSequence<AnyIterator<[Int]>, [Element]> {
    return encode().prefixesInt().lazy.map { $0.decode(self) }
  }
}

for prefix in ["H","A","T"].prefixes() {
  print(prefix)
}

extension Array where Element: Equatable {
  func encodeTransformDecode(transform: ([Int]) -> AnyIterator<[Int]> ) -> LazyMapSequence<AnyIterator<[Int]>, [Element]> {
    return transform(encode()).lazy.map { $0.decode(self) }
  }
}

print("============ prefixes2")
extension Array where Element: Equatable {
  func prefixes2() -> LazyMapSequence<AnyIterator<[Element]>, [Element]> {
    return encodeTransformDecode(transform: prefixesInt)
  }
}

for prefix in ["A","B","C"].prefixes2() {
  print(prefix)
}