带有 andThen 类型不匹配的 Scala 链接函数 [英] Scala chaining functions with andThen type mismatch

问题描述

我有很多功能可以清理文本并将它们拆分为单词.最小示例:

I have a bunch of functions that clean text and splits them into words. Minimal example:

val txt = "Mary had a @little \nlamb"
val stopwords = Seq("a")
def clean(text: String): String = text.replaceAll("\n*\r*", "")
def tokenize(text: String): Seq[String] = text.split("\\s")

val cleaned = clean(txt)
val tokens = tokenize(cleaned)

此代码按预期工作.然而并不是很地道.我曾希望这样做:

This code works as expected. However not really idiomatic. I had hoped to do this:

clean(txt) andThen tokenize

但是编译器抱怨这个错误 type mismatch;要求:字符 =>? 在 tokenize 函数中.

But the compiler complains about this with the error type mismatch; required: Char => ? at the tokenize function.

我在这里遗漏了什么?

推荐答案

clean 返回一个 String.您正在尝试在 String 实例上使用 andThen(因为您正在使用 clean(txt)methodcode>) 并且编译器将其推断为 PartialFunction[Char, ?](因为 WrappedString 继承了 AbstractSeq[Char]，它继承了 PartialFunction[Char, A]).这就是您看到类型不匹配的原因.如果要将两者组合在一起，请使用 eta-expansion 将它们转换为函数类型:

clean returns a String. You're trying to use andThen on a String instance (since you're invoking the method with clean(txt)) and the compiler infers it as a PartialFunction[Char, ?] (because WrappedString inherits AbstractSeq[Char] which inherits PartialFunction[Char, A]). That is why you're seeing the type mismatch. If you want to compose the two together, turn them into function types using eta-expansion:

val res = clean _ andThen tokenize
println(res(txt))

函数组合适用于 Scala 函数，而不是 方法(有区别)，这就是为什么我们必须首先将方法扩展为函数(clean _)，然后编译器就可以为我们推断出tokenize，而无需手动扩展它.

Function composition works on Scala functions, not methods (there is a distinction), and that is why we have to first expand the method to a function (clean _), and then the compiler will be able to infer tokenize for us without the need to manually expand it.

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