组合和然后方法 [英] Compose and andThen methods

问题描述

我正在学习教程 模式匹配 &Scala compose 和 andThen 方法上的函数组合.有这样一个例子:

I'm following the tutorial Pattern matching & functional composition on Scala compose and andThen methods. There's such an example:

scala> def addUmm(x: String) = x + " umm"
scala> def addAhem(x: String) = x + " ahem"

val ummThenAhem = addAhem(_).compose(addUmm(_))

当我尝试使用它时出现错误:

When I try to use it I get an error:

<console>:7: error: missing parameter type for expanded function ((x$1) => addAhem(x$1).compose(((x$2) => addUmm(x$2))))
   val ummThenAhem = addAhem(_).compose(addUmm(_))
                             ^
<console>:7: error: missing parameter type for expanded function ((x$2) => addUmm(x$2))
   val ummThenAhem = addAhem(_).compose(addUmm(_))
                                               ^
<console>:7: error: type mismatch;
 found   : java.lang.String
 required: Int
     val ummThenAhem = addAhem(_).compose(addUmm(_))

但是，这是有效的:

val ummThenAhem = addAhem _ compose addUmm _

甚至

val ummThenAhem = addAhem _ compose addUmm

教程中的代码有什么问题?后一个表达式不是和第一个没有括号的表达式一样吗?

What's wrong with the code in the tutorial? Isn't the latter expression the same as the first one without parenthesis?

推荐答案

addAhem 是一种方法.compose 方法定义在函数上.addAhem _ 将 addAhem 从方法转换为函数，因此可以在其上调用 compose.compose 需要一个函数作为参数.您通过将 addUmm 转换为带有 addUmm _ 的函数(下划线可以省略，因为编译器可以自动转换当它知道无论如何都需要一个函数时将方法转换为函数).所以你的代码:

addAhem is a method. compose method is defined on functions. addAhem _ converts addAhem from method to function, so compose can be called on it. compose expects a function as it's argument. You are giving it a method addUmm by converting addUmm into a function with addUmm _ (The underscore can be left out because the compiler can automatically convert a method into a function when it knows that a function is expected anyway). So your code:

addAhem _ compose addUmm

与

(addAhem _).compose(addUmm)

但不是

addAhem(_).compose(addUmm(_))

附注我没有看你提供的链接.

PS I didn't look at the link you provided.

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