斯卡拉有警卫吗？ [英] Does Scala have guards?

问题描述

我几天前开始学习scala，在学习它时，我将它与其他函数式编程类似的语言（ Haskell ， Erlang ），我对它有一定的了解。 Scala是否有守卫序列？

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中进行了模式匹配，但是有没有相当于使用的守卫的概念，否则和所有？

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，如果使用关键字。来自斯卡拉A Tour之旅的案例分类部分，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，也许是因为Tour是如此快速的概述。）

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

在Haskell中，否则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添加任何力量。你可以通过重复你的初始模式而没有后卫来获得它：

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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