当ElementOfResult被推断为可选时，flatMap不会过滤掉nil

本文介绍了当ElementOfResult被推断为可选时，flatMap不会过滤掉nil的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述 Swift的flatMap文档读取： 设置 resultTypeSpecified 到 [String？] ，你告诉编译器 ElementOfResult 是可选<字符串> 。 变换闭包的类型为（String？） - >任选的LT;任选的LT;字符串>> 。 flatMap 将带走1层可选层，而不是2层。 b 希望这个例子能够让事情变得更加清晰： let input：[String ??] = [ Optional.some（Optional.some（1989））， Optional.some（Optional.none）， Optional.some（Optional.some（Fearless））， Optional.some（Optional.none）， Optional.some（Optional.some（Red））] let output = input.flatMap（{ $ 0}） Swift documentation of flatMap reads: In the following examples when return type of ElementOfResult is left to the compiler to infer flatMap works as documented, yet on line 5 when ElementOfResult is specified, thus inferred to an Optional<String> type it seems that flatMap stops filtering out nil's. Why is it doing that?~ swiftWelcome to Apple Swift version 3.0.2 (swiftlang-800.0.63 clang-800.0.42.1). Type :help for assistance. 1> let words = ["1989", nil, "Fearless", nil, "Red"]words: [String?] = 5 values { [0] = "1989" [1] = nil [2] = "Fearless" [3] = nil [4] = "Red"} 2> words.flatMap { $0 }$R0: [String] = 3 values { [0] = "1989" [1] = "Fearless" [2] = "Red"} 3> let resultTypeInferred = words.flatMap { $0 }resultTypeInferred: [String] = 3 values { [0] = "1989" [1] = "Fearless" [2] = "Red"} 4> let resultTypeSpecified: [String?] = words.flatMap { $0 }resultTypeSpecified: [String?] = 5 values { [0] = "1989" [1] = nil [2] = "Fearless" [3] = nil [4] = "Red"} 解决方案 Here's the definition of flatMap()When you set the type of resultTypeSpecified to [String?], you tell the compiler that ElementOfResult is Optional<String>.Your transform closure has a type of (String?) -> Optional<Optional<String>>. flatMap will take away 1 "layer" of optionals but not 2 layers.Hopefully this example will makes things clearer:let input: [String??] = [ Optional.some(Optional.some("1989")), Optional.some(Optional.none), Optional.some(Optional.some("Fearless")), Optional.some(Optional.none), Optional.some(Optional.some("Red"))]let output = input.flatMap({ $0 }) 这篇关于当ElementOfResult被推断为可选时，flatMap不会过滤掉nil的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！