scala 类似于 ? 的三元运算符：

Ternary Operator Similar To ?:

提问by Peter Schmitz

I am trying to avoid constructs like this:

我试图避免这样的结构：

val result = this.getClass.getSimpleName
if (result.endsWith("$")) result.init else result

Ok, in this example the thenand elsebranch are simple, but you can image complex ones. I built the following:

好的，在这个例子中，then和else分支很简单，但你可以想象复杂的。我构建了以下内容：

object TernaryOp {
  class Ternary[T](t: T) {
    def is[R](bte: BranchThenElse[T,R]) = if (bte.branch(t)) bte.then(t) else bte.elze(t)
  }
  class Branch[T](branch: T => Boolean) {
    def ?[R] (then: T => R) = new BranchThen(branch,then)
  }
  class BranchThen[T,R](val branch: T => Boolean, val then: T => R)
  class Elze[T,R](elze: T => R) {
    def :: (bt: BranchThen[T,R]) = new BranchThenElse(bt.branch,bt.then,elze)
  }
  class BranchThenElse[T,R](val branch: T => Boolean, val then: T => R, val elze: T => R)
  implicit def any2Ternary[T](t: T) = new Ternary(t)
  implicit def fct2Branch[T](branch: T => Boolean) = new Branch(branch)
  implicit def fct2Elze[T,R](elze: T => R) = new Elze(elze)
}

Defined that, I can replace the above simple example with:

定义为，我可以将上面的简单示例替换为：

this.getClass.getSimpleName is {s: String => s.endsWith("$")} ? {s: String => s.init} :: {s: String => s}

But how can I get rid of the s: String =>? I want something like that:

但是我怎样才能摆脱s: String =>？我想要这样的东西：

this.getClass.getSimpleName is {_.endsWith("$")} ? {_.init} :: {identity}

I guess the compiler needs the extra stuff to infer types.

我猜编译器需要额外的东西来推断类型。

采纳答案by Rex Kerr

We can combine How to define a ternary operator in Scala which preserves leading tokens?with the answer to Is Option wrapping a value a good pattern?to get

我们可以结合如何在 Scala 中定义一个保留前导标记的三元运算符？答案是 Option wrapping a value a good pattern? 要得到

scala>   "Hi".getClass.getSimpleName |> {x => x.endsWith("$") ? x.init | x}
res0: String = String

scala> List.getClass.getSimpleName |> {x => x.endsWith("$") ? x.init | x}
res1: String = List

Is this adequate for your needs?

这是否足以满足您的需求？

回答by Landei

From Tony Morris' Lambda Blog:

来自Tony Morris 的 Lambda 博客：

I hear this question a lot. Yes it does. Instead of c ? p : q, it is written if(c) p else q.

This may not be preferable. Perhaps you'd like to write it using the same syntax as Java. Sadly, you can't. This is because :is not a valid identifier. Fear not, |is! Would you settle for this?

c ? p | q

Then you'll need the following code. Notice the call-by-name (=>) annotations on the arguments. This evaluation strategy is required to correctly rewrite Java's ternary operator. This cannot be done in Java itself.

case class Bool(b: Boolean) {   
  def ?[X](t: => X) = new {
    def |(f: => X) = if(b) t else f   
  } 
}

object Bool {   
  implicit def BooleanBool(b: Boolean) = Bool(b) 
}

Here is an example using the new operator that we just defined:

object T {   val condition = true

  import Bool._

  // yay!   
  val x = condition ? "yes" | "no"
}

Have fun ;)

我经常听到这个问题。是的，它确实。相反c ? p : q，它被写入if(c) p else q。

这可能不是优选的。也许您想使用与 Java 相同的语法来编写它。可悲的是，你不能。这是因为:不是有效的标识符。不要害怕，|是！你愿意接受这个吗？

c ? p | q

然后你需要下面的代码。注意参数上的 call-by-name ( =>) 注释。要正确重写 Java 的三元运算符，需要此评估策略。这不能在 Java 本身中完成。

case class Bool(b: Boolean) {   
  def ?[X](t: => X) = new {
    def |(f: => X) = if(b) t else f   
  } 
}

object Bool {   
  implicit def BooleanBool(b: Boolean) = Bool(b) 
}

这是一个使用我们刚刚定义的 new 运算符的示例：

object T {   val condition = true

  import Bool._

  // yay!   
  val x = condition ? "yes" | "no"
}

玩得开心 ;）

回答by Debilski

Rex Kerr's answerexpressed in basic Scala:

Rex Kerr在基本 Scala 中的回答：

"Hi".getClass.getSimpleName match {
  case x if x.endsWith("$") => x.init
  case x => x
}

although I'm not sure what part of the if–else construct you want to optimise.

虽然我不确定您想要优化 if-else 结构的哪一部分。

回答by Wouter

Since if-else constructions in Scala return a value, you can use this

由于 Scala 中的 if-else 构造返回一个值，因此您可以使用它

val a = if (1 < 0) 1 else 2

More info: https://alvinalexander.com/scala/scala-if-then-ternary-operator-cookbook-examples

更多信息：https: //alvinalexander.com/scala/scala-if-then-ternary-operator-cookbook-examples

回答by Ustaman Sangat

Since : by itself won't be a valid operator unless you are ok with always escaping it with back ticks :, you could go with another character, e.g. "|" as in one of the answers above. But how about elvis with a goatee ?::

因为 : 本身不会是一个有效的运算符，除非你总是用反勾号转义它:，你可以使用另一个字符，例如“|” 如上述答案之一。但是留着山羊胡子的猫王呢？：：

implicit class Question[T](predicate: => Boolean) {
  def ?(left: => T) = predicate -> left
}
implicit class Colon[R](right: => R) {
  def ::[L <% R](pair: (Boolean, L)): R = if (q._1) q._2 else right
}
val x = (5 % 2 == 0) ? 5 :: 4.5

Of course this again won't work if you values are lists, since they have :: operator themselves.

当然，如果您的值是列表，这将再次不起作用，因为它们本身具有 :: 运算符。