scala - 如何在 Scala 中接受多种类型

Question

我有一些接受多种类型并返回相同类型的重载方法：

def foo(x: Int): Foo = ... 
def foo(x: String): Foo = ... 
def foo(x: Boolean): Foo = ... 
def foo(x: Long): Foo = ...

现在我想定义一种调用该方法的方法，例如：

def bar(x: Int | String | Boolean | Long) = foo(x)  // how to do this? 

我可以用我不太喜欢的“天真”方式来做到这一点：

def bar(x: Any) = x match {
  case i:Int => foo(i)
  case s:String => foo(s)
  case b:Boolean => foo(b)
  case l:Long => foo(l)
  case _ => throw new Exception("Unsupported type")
}

有没有更好的方法，也许使用 Scalaz 或其他一些库？

标签： scala

Answer 1

尝试类型类

trait FooDoer[T] {
  def foo(x: T): Foo
}
object FooDoer {
  implicit val int: FooDoer[Int] = (x: Int) => foo(x)
  implicit val string: FooDoer[String] = (x: String) => foo(x)
  implicit val boolean: FooDoer[Boolean] = (x: Boolean) => foo(x)
  implicit val long: FooDoer[Long] = (x: Long) => foo(x)
}

def bar[T](x: T)(implicit fooDoer: FooDoer[T]): Foo = fooDoer.foo(x)

bar(1)
bar("a")
bar(true)
bar(1L)
// bar(1.0) // doesn't compile

有时以下内容也会有所帮助

def bar[T](x: T)(implicit ev: (T =:= Int) | (T =:= String) | (T =:= Boolean) | (T =:= Long)) = ???

trait |[A, B]
trait LowPriority_| {
  implicit def a[A, B](implicit a: A): A | B = null
}
object | extends LowPriority_| {
  implicit def b[A, B](implicit b: B): A | B = null
}

如何定义“类型析取”（联合类型）？