寻找将在Scala 2.7.7和2.8脚本中使用的类型类用法

Question

嗨，我正在寻找Scala中类型类用法的快速示例，该类型将在2.7.7和2.8环境中起作用。

我看到的所有示例仅在2.8中起作用，但是我被告知该类型类也可以在2.7.7中实现。

唯一的其他要求是使用情况必须能够在脚本中工作。

任何例子都赞赏！谢谢

Answer 1

我沿着这些方案采取了解决方案：

//---------- Classes without needed behavior
case class Point(x: Double, y: Double)
trait Shape {}
case class Circle(center: Point, radius: Double) extends Shape {}
case class Rectangle(lowerLeft: Point, upperRight: Point) extends Shape {}


val circle = Circle(Point(1.0, 2.0), 5.0)
val rectangle = Rectangle(Point(-2.0, -1.0), Point(4.0, 5.0))


//----------- Support for new behavior I want to add

// Create exception to handle non supported subclasses
case class NewMethodNotSupported(a: Any)
    extends RuntimeException(a.toString + ".newMethod() not supported")

class NewBehavior(shape: Shape) {
    def newMethod() = shape match {
        case c: Circle => doMethod(c)
        case r: Rectangle => doMethod(r)
        case _ => throw NewMethodNotSupported(shape)
    }
    private
    def doMethod(s: Shape) = println(s) // print to standard out.
}

object NewBehavior {
    // invoked by the compiler
    implicit def shapeToNewBehavior(s: Shape) = new NewBehavior(s)
}

// bring the implicit method in scope
import NewBehavior._
// --------- End of new behavior support        


// Test behavior support:
println("Test that new behavior is added:")
circle.newMethod()
rectangle.newMethod()

// Create a non supported shape class
case class Triangle(vertex1: Point,
        vertex2: Point, vertex3: Point) extends Shape {}

val triangle = Triangle(Point(-1.0,0.0), Point(1.0,0.0), Point(0.0,1.0))

// Catch exception thrown by trying to call method from unsupported shape
try{    
    println("\nTest method call from unsupported shape:")
    triangle.newMethod()
} catch {
    case dns: NewMethodNotSupported => println(dns); System.exit(-1)
    case unknown => println("Uknown exception " + unknown); System.exit(-1)
}

Answer 2

我想你可以做这样的事情：

def max[A](list: List[A])(implicit ord: Ordering[A]): A = {
  list.tail.foldLeft(list.head) ((a, b) => if (ord.lt(a, b)) b else a)
}

implicit def toOrdering[A <% Ordered[A]]: Ordering[A] = new Ordering[A] {
    def compare(a: A, b: A): Int = (a < b, b < a) match {
        case (true, _) => -1
        case (_, true) => 1
        case _ => 0
    }
}

println(max(List(1, 2, 3, 2, 1)))

该代码在Scala 2.7.7和2.8.0（现在都在两者中都进行了测试），尽管隐式定义在Scala 2.8中是不必要的（在某些情况下可能有害的）。