我可以在 Scala 匹配语句中使用类变量吗？

发布于 2024-09-30 14:37:28 字数 657 浏览 1 评论 0原文

假设我有这样的事情：

obj match {
    case objTypeOne : TypeOne => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

现在我想概括一下，传递一种类型来匹配：

obj match {
    case objTypeOne : clazz => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

但这是不允许的，我认为是出于语法而不是语义的原因（尽管我也猜想，尽管 clazz 是Class[C] 类型被擦除，因此选项的类型将丢失）。

我最后的结论是：

if(clazzOne.isAssignableFrom(obj.getClass)) Some(clazz.cast(obj))
if(obj.isInstanceOf[TypeTwo]) Some(obj.asInstanceOf[TypeTwo])
None

我只是想知道是否有更好的方法。

原文

Say I have something like this:

obj match {
    case objTypeOne : TypeOne => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

Now I want to generalise, to pass in one of the types to match:

obj match {
    case objTypeOne : clazz => Some(objTypeOne)
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case _ => None
}

But this isn't allowed, I think for syntactic rather than semantic reasons (although I guess also that even though the clazz is a Class[C] the type is erased and so the type of the Option will be lost).

I ended up with:

if(clazzOne.isAssignableFrom(obj.getClass)) Some(clazz.cast(obj))
if(obj.isInstanceOf[TypeTwo]) Some(obj.asInstanceOf[TypeTwo])
None

I just wondered if there was a nicer way.

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逆光飞翔i 2024-10-07 14:37:28

您可以定义一个提取器来匹配您的对象：

class IsClass[T: Manifest] {
  def unapply(any: Any): Option[T] = {
    if (implicitly[Manifest[T]].erasure.isInstance(any)) {
       Some(any.asInstanceOf[T])
    } else {
       None
    }
  }
}

所以让我们测试一下：

class Base { def baseMethod = () }
class Derived extends Base

val IsBase = new IsClass[Base]

def test(a:Any) = a match {
    case IsBase(b) => 
      println("base")
      b.baseMethod
    case _ => println("?")
  }

test(new Base)
test(1)

您必须为提取器定义一个 val，例如，您不能内联 IsBase。否则它会被解释为提取器。

You could define an extractor to match your object:

class IsClass[T: Manifest] {
  def unapply(any: Any): Option[T] = {
    if (implicitly[Manifest[T]].erasure.isInstance(any)) {
       Some(any.asInstanceOf[T])
    } else {
       None
    }
  }
}

So let's test it:

class Base { def baseMethod = () }
class Derived extends Base

val IsBase = new IsClass[Base]

def test(a:Any) = a match {
    case IsBase(b) => 
      println("base")
      b.baseMethod
    case _ => println("?")
  }

test(new Base)
test(1)

You will have to define a val for your extractor, you can't inline IsBase, for example. Otherwise it would be interpreted as an extractor.

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星星的軌跡 2024-10-07 14:37:28

您可以使用模式防护来实现这一点。尝试这样的事情：

obj match {
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case objTypeOne if clazz.isAssignableFrom(objTypeOne.getClass) => Some(clazz.cast(objTypeOne))
    case _ => None
}

You could use pattern guards to achieve that. Try something like this:

obj match {
    case objTypeTwo : TypeTwo => Some(objTypeTwo)
    case objTypeOne if clazz.isAssignableFrom(objTypeOne.getClass) => Some(clazz.cast(objTypeOne))
    case _ => None
}

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旧时模样 2024-10-07 14:37:28

~~您可以使用本地类型别名：~~

def matcher[T](obj: Any)(implicit man: Manifest[T]) = { val instance = man.erasure.newInstance.asInstanceOf[AnyRef] type T = instance.type // type alias obj match { case objTypeOne : T => "a" case objTypeTwo : TypeTwo => "b" case _ => "c" } } scala> matcher[TypeOne](TypeOne()) res108: java.lang.String = a scala> matcher[TypeTwo](TypeOne()) res109: java.lang.String = c

更新： Aaron Novstrup 指出，单例类型仅在 man.erasure.newInstance==obj 时才有效（请参阅规范的 §3.2.1）

~~You can use a local type alias for that:~~

UPDATE: Aaron Novstrup has pointed out that singleton type will only work if man.erasure.newInstance==obj (see §3.2.1 of the spec)

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