Scala 大小写匹配默认值
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Scala case match default value
提问by Caballero
How can I get the default value in match case?
如何在匹配案例中获得默认值？
//Just an example, this value is usually not known
val something: String = "value"
something match {
case "val" => "default"
case _ => smth(_) //need to reference the value here - doesn't work
}
UPDATE:I see that my issue was not really understood, which is why I'm showing an example which is closer to the real thing I'm working on:
更新：我发现我的问题没有被真正理解，这就是为什么我要展示一个更接近我正在研究的真实事物的例子：
val db = current.configuration.getList("instance").get.unwrapped()
.map(f => f.asInstanceOf[java.util.HashMap[String, String]].toMap)
.find(el => el("url").contains(referer))
.getOrElse(Map("config" -> ""))
.get("config").get match {
case "" => current.configuration.getString("database").getOrElse("defaultDatabase")
case _ => doSomethingWithDefault(_)
}
回答by om-nom-nom
something match {
case "val" => "default"
case default => smth(default)
}
It is not a keyword, just an alias, so this will work as well:
它不是关键字，只是别名，所以这也可以：
something match {
case "val" => "default"
case everythingElse => smth(everythingElse)
}
回答by Joyfulvillage
The "_" in Scala is a love-and-hate syntax which could really useful and yet confusing.
Scala 中的“_”是一种又爱又恨的语法，它可能非常有用但又令人困惑。
In your example:
在你的例子中：
something match {
case "val" => "default"
case _ => smth(_) //need to reference the value here - doesn't work
}
the _ means, I don't care about the value, as well as the type, which means you can't reference to the identifier anymore.
Therefore, smth(_) would not have a proper reference.
The solution is that you can give the a name to the identifier like:
_ 表示，我不关心值以及类型，这意味着您不能再引用标识符。因此， smth(_) 不会有正确的引用。
解决方案是您可以为标识符指定一个名称，例如：
something match {
case "val" => "default"
case x => smth(x)
}
I believe this is a working syntax and x will match any value but not "val".
我相信这是一种有效的语法，x 将匹配任何值，但不匹配“val”。
More speaking. I think you are confused with the usage of underscore in map, flatmap, for example.
说多了。例如，我认为您对 map、flatmap 中下划线的使用感到困惑。
val mylist = List(1, 2, 3)
mylist map { println(_) }
Where the underscore here is referencing to the iterable item in the collection. Of course, this underscore could even be taken as:
这里的下划线是指集合中的可迭代项。当然，这个下划线甚至可以理解为：
mylist map { println }
回答by Dan
here's another option:
这是另一种选择：
something match {
case "val" => "default"
case default@_ => smth(default)
}
