[英]Scala optional pattern matching
我发现自己经常使用模式匹配,返回一个Option
,其中无匹配大小写返回None
,例如
x match {
case A(a) => Some(a)
case B(b) => Some(b)
case _ => None
}
我可以想象使用它来简化这个
object MaybeMatchImplicits {
implicit class MaybeMatcher[A](val underlying: A) extends AnyVal {
@inline
def maybeMatch[B](f: PartialFunction[A, B]): Option[B] = f.lift.apply(underlying)
}
}
这使得
scala> import MaybeMatchImplicits._
import MaybeMatchImplicits._
scala> 5 maybeMatch { case 5 => 'good }
res0: Option[Symbol] = Some('good)
scala> 6 maybeMatch { case 5 => 'good }
res1: Option[Symbol] = None
我想知道这种方法是否隐藏任何陷阱和/或在Scala 2.11+中是否有更简单/更好/更惯用的机制。
更新:目标是处理匹配的rhs上的任意计算,这使得基于异常的解决方案不合需要。
成语:
scala> case class A(a: Int) ; case class B(b: String)
defined class A
defined class B
scala> def f(x: Any) = Option(x) collect { case A(a) => a ; case B(b) => b }
f: (x: Any)Option[Any]
scala> f(42)
res0: Option[Any] = None
scala> f(A(42))
res1: Option[Any] = Some(42)
scala> f(B("ok"))
res2: Option[Any] = Some(ok)
或者:
scala> import PartialFunction.{cond => when, condOpt => whenever}
import PartialFunction.{cond=>when, condOpt=>whenever}
scala> def f(x: Any) = whenever(x) { case A(a) => a ; case B(b) => b }
f: (x: Any)Option[Any]
scala> f(42)
res3: Option[Any] = None
scala> f(A(42))
res4: Option[Any] = Some(42)
scala> f(B("ok"))
res5: Option[Any] = Some(ok)
使用get
方法(请参阅下面给出的实现),该方法将给定值包装在选项周围,然后收集所需的值。
使用选项包装值,然后收集您想要收集的内容。
Option(x: Any).collect { case 1 => 1 }
要么
x get { case 2 => 2 } // get implementation is given below
Scala REPL
scala> Option(1).collect { case 1 => 1 }
res0: Option[Int] = Some(1)
scala> Option(2).collect { case str: String => "bad" }
<console>:12: error: scrutinee is incompatible with pattern type;
found : String
required: Int
Option(2).collect { case str: String => "bad" }
^
scala> Option(2: Any).collect { case str: String => "bad" }
res2: Option[String] = None
scala> Option(2: Any).collect { case 2 => "bad" }
res3: Option[String] = Some(bad)
implicit class InnerValue[A](value: A) {
def get[B](pf: PartialFunction[Any, B]): Option[B] = Option(value) collect pf
}
Scala REPL
scala> implicit class InnerValue[A](value: A) {
| def get[B](pf: PartialFunction[Any, B]): Option[B] = Option(value) collect pf
| }
defined class InnerValue
scala> 2.get { case 2 => 2}
res5: Option[Int] = Some(2)
scala> 2.get { case 3 => 2}
res6: Option[Int] = None
现在您可以invoke
get方法并传递partial函数。 现在,您可能会获得包含在Some中的值或将获得None。
请注意,上面的API(get方法)不是类型安全的,您可以这样做
2.get { case str: String => str }
哪个返回None。
现在,如果您想要类型安全,请进行以下更改
implicit class InnerValue[A](value: A) {
def get[B](pf: PartialFunction[A, B]): Option[B] = Option(value) collect pf
}
请注意,在partial函数中,输入参数类型是A
而不是Any。
现在,当你这样做
2.get { case str: String => str }
您将收到编译错误。
scala> 2.get { case str: String => str }
<console>:15: error: scrutinee is incompatible with pattern type;
found : String
required: Int
2.get { case str: String => str }
您可以通过执行以下操作来解决编译错误
scala> (2: Any) get { case str: String => str}
res16: Option[String] = None
我会这样做:
def trial(a:Any, f:Any => Any) = try Some(f(a)) catch {case _:MatchError => None}
implicit class BetterAny(a:Any) {
def betterMatch(f:Any => Any) = trial(a, f)
}
// some example classes
case class A(i:Int)
case class B(s:String)
在REPL上:
scala> A(1) betterMatch {case A(a) => a; case B(b) => b}
res11: Option[Any] = Some(1)
scala> 2 betterMatch {case A(a) => a; case B(b) => b}
res12: Option[Any] = None
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