Scala 有守卫吗? [英] Does Scala have guards?

问题描述

几天前我开始学习scala，在学习它时，我将它与其他函数式编程进行了比较 语言，如 (Haskell、Erlang)，我对它有些熟悉.Scala 是否有可用的guard 序列?

I started learning scala a few days ago and when learning it, I am comparing it with other functional programming languages like (Haskell, Erlang) which I had some familiarity with. Does Scala has guard sequences available?

我在 Scala 中经历了模式匹配，但是否有任何概念等同于 otherwise 和 all 的守卫?

I went through pattern matching in Scala, but is there any concept equivalent to guards with otherwise and all?

推荐答案

是的，它使用关键字 if.来自 Scala 之旅的案例类部分，靠近底部:

Yes, it uses the keyword if. From the Case Classes section of A Tour of Scala, near the bottom:

def isIdentityFun(term: Term): Boolean = term match {
  case Fun(x, Var(y)) if x == y => true
  case _ => false
}

(模式匹配页面上没有提到这一点，可能是因为 Tour是一个如此快速的概述.)

(This isn't mentioned on the Pattern Matching page, maybe because the Tour is such a quick overview.)

在 Haskell 中，otherwise 实际上只是一个绑定到 True 的变量.所以它不会为模式匹配的概念增加任何力量.你可以通过在没有守卫的情况下重复你的初始模式来获得它:

In Haskell, otherwise is actually just a variable bound to True. So it doesn't add any power to the concept of pattern matching. You can get it just by repeating your initial pattern without the guard:

// if this is your guarded match
  case Fun(x, Var(y)) if x == y => true
// and this is your 'otherwise' match
  case Fun(x, Var(y)) if true => false
// you could just write this:
  case Fun(x, Var(y)) => false

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