需要函数参数实现方法 - Scala

Question

有没有一种方法可以要求传递给函数的对象实现一组核心方法？

例如，我希望能够编写一个求和方法来对实现“+”运算符的任何可迭代对象求和。

我的初步实现如下

trait addable[T <: addable[T]]{
     def +(other: T): T
 }

 def sum[T <: addable[T]](items: Iterable[T]) = 
     if(items.isEmpty) throw new Exception("Can't sum nothing")
     else items.tail.foldRight(items.head)(_+_) 
          //Starst with the first element and adds all other elements to it

现在这个方法行得通，但是很笨拙。如果我想让某些东西可以求和，我必须在我想求和的每个 class 中显式实现 addable[T]，更不用说为数字类型和字符串定义一堆显式转换了。

有没有办法让它看起来像这样？

def sum[T fulfills addable[T]](items: Iterable[T]) = 
    if(items.isEmpty) throw new Exception("Can't sum nothing")
    else items.tail.foldRight(items.head)(_+_)

或者，是否有一些设计模式可以消除对此的需求（我现在有效地做的似乎只是适配器模式）？

Answer 1

做这种事情的常见模式是使用类型类：http://typelevel.org/cats/typeclasses.html

这是您的用例的 Addable 类型类的示例实现：

trait Addable[T] {
  def +(a: T, b: T): T
}

// Among other places Scala searches for implicits 
// in the companion objects of the relevant classes.
// Read more in this answer: 
object Addable {

  // Using context bound notation
  def apply[T : Addable]: Addable[T] = implicitly

  // Instance of Addable typeclass for types,
  // that have an instance of the built-in Numeric typeclass
  implicit def numeric[T : Numeric]: Addable[T] = {
    import Numeric.Implicits._
    // This uses Scala 2.12 feature of automatic convertions of lambdas to SAMs
    // You can create an instance of an anonymous subclass in older versions.
    _ + _ 
  }

  // Instance of Addable for all kinds of Iterables, 
  // that adds them element by element (like mathematical vectors)
  implicit def iterable[That, T](implicit
    ev: That <:< IterableLike[T, That], // To compute the element type T from That
    cbf: CanBuildFrom[That, T, That],   // To call `map` method
    add: Addable[T]                     // To add elements of the iterable
  ): Addable[That] =
    (a, b) => (a, b).zipped.map(add.+)
}

这里是 sum 方法的示例实现，它使用了这个 Addable 类型类：

def sum[T : Addable](items: Iterable[T]): T = items.
  reduceOption(Addable[T].+).
  getOrElse(throw new Exception("Can't sum nothing"))

以及一些使用它的结果：

scala> sum(Seq(1.2, 3.4, 5.6))
res0: Double = 10.2

scala> sum(Seq(Vector(1,2), Vector(4,5), Vector(6,7)))
res1: scala.collection.immutable.Vector[Int] = Vector(11, 14)

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