Scala 2.10 中的字符串插值 - 如何插入字符串变量? [英] String interpolation in Scala 2.10 - How to interpolate a String variable?

问题描述

从 Scala 2.10 开始，字符串插值 在 Scala 中可用

String interpolation is available in Scala starting Scala 2.10

这是一个基本的例子

 val name = "World"            //> name  : String = World
 val message = s"Hello $name"  //> message  : String = Hello World

我想知道是否有办法进行动态插值，例如以下(不编译，仅用于说明目的)

I was wondering if there is a way to do dynamic interpolation, e.g. the following (doesn't compile, just for illustration purposes)

 val name = "World"            //> name  : String = World
 val template = "Hello $name"  //> template  : String = Hello $name
 //just for illustration:
 val message = s(template)     //> doesn't compile (not found: value s)

1. 有没有办法动态"评估这样的字符串?(或者它本质上是错误的/不可能的)

1. Is there a way to "dynamically" evaluate a String like that? (or is it inherently wrong / not possible)

s 究竟是什么?这不是方法定义 (显然它是 StringContext 上的一个方法)，而不是一个对象(如果是，它会抛出一个不同的编译错误比未找到我认为)

And what is s exactly? it's not a method def (apparently it is a method on StringContext), and not an object (if it was, it would have thrown a different compile error than not found I think)

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推荐答案

s 实际上是 StringContext 上的一个方法(或者可以从 StringContext).当你写

s is actually a method on StringContext (or something which can be implicitly converted from StringContext). When you write

whatever"Here is text $identifier and more text"

编译器将其脱糖

StringContext("Here is text ", " and more text").whatever(identifier)

默认情况下，StringContext 为您提供 s、f 和 raw* 方法.

By default, StringContext gives you s, f, and raw* methods.

如您所见，编译器自己挑选名称并将其提供给方法.由于这是在编译时发生的，因此您无法明智地动态执行此操作——编译器在运行时没有关于变量名称的信息.

As you can see, the compiler itself picks out the name and gives it to the method. Since this happens at compile time, you can't sensibly do it dynamically--the compiler doesn't have information about variable names at runtime.

但是，您可以使用变量，因此您可以交换所需的值.默认的 s 方法只调用 toString(正如你所期望的)，所以你可以玩像

You can use vars, however, so you can swap in values that you want. And the default s method just calls toString (as you'd expect) so you can play games like

class PrintCounter {
  var i = 0
  override def toString = { val ans = i.toString; i += 1; ans }
}

val pc = new PrintCounter
def pr[A](a: A) { println(s"$pc: $a") }
scala> List("salmon","herring").foreach(pr)
1: salmon
2: herring

(在这个例子中 0 已经被 REPL 调用了).

(0 was already called by the REPL in this example).

这大概是你能做的最好的了.

That's about the best you can do.

*^{raw 已损坏，预计要到 2.10.1 才能修复；只有变量之前的文本实际上是原始的(没有转义处理).所以在 2.10.1 发布之前不要使用那个，或者查看源代码并定义你自己的.默认情况下，没有转义处理，因此定义您自己的处理非常容易.}

*^{raw is broken and isn't slated to be fixed until 2.10.1; only text before a variable is actually raw (no escape processing). So hold off on using that one until 2.10.1 is out, or look at the source code and define your own. By default, there is no escape processing, so defining your own is pretty easy.}

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