在Scala中对泛型类实现二进制操作

Question

I am having multiple compile errors trying to write a class definition like this.

    trait BinOps[T] {
      def +(that: Vector[T]): Vector[T]
      def -(that: Vector[T]): Vector[T]
      def *(that: Vector[T]): Vector[T]
      def /(that: Vector[T]): Vector[T]
    }

    trait Vector[T] {
      def toList(): List[T]
      def zeros(length: Int): Vector[T]
    }

    object Vector {
      def apply[T](args: T*): Vector[T] = new VectorImpl[T](args.toList)

      private class VectorImpl[@specialized(Double, Int, Float, Long)T](val _data: List[T])
        extends Vector[T]
        with BinOps[T] {
        def +(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 + elem._2))
        def -(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 - elem._2))
        def *(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 * elem._2))
        def /(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 / elem._2))

        def fill(length: Int, value: T): Vector[T] = new VectorImpl[T](List.fill[T](length)(value))
        def toList(): List[T] = _data.toList
      }

      implicit def VectorToList[T](v: Vector[T]): List[T] = v.toList
    }

I am getting errors like these..

    Error:(36, 102) type mismatch;
     found   : T
     required: String
        def +(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 + elem._2))
                                                                         ^   
    Error:(37, 95) value - is not a member of type parameter T
        def -(that: Vector[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 - elem._2))
                                                                         ^

However doing a zip should have resulted in a tuple with type T and I am trying to access the first and second elements. So what am I doing wrong ?

Answer 1

Error:(37, 95) value - is not a member of type parameter T

This is the real error (the one about strings is an unfortunate consequence of an implicit conversion in Predef - I recommend building with -Yno-predef to avoid such misleading errors). Your T is unconstrained - what is - supposed to do? Eg what if T was Locale - what is Locale.ENGLISH - Locale.SPANISH ? So it doesn't compile.

Answer 2

All the answers above to point to the right problem. I will post the answer of what finally worked. As suggested by JimN and from help from the scala irc this worked for me finally. Thanks a lot for the explanations

import Numeric.Implicits._
import Fractional.Implicits._


trait BinOps[T] {
  def +(that: Vector[T])(implicit ev: Numeric[T]): Vector[T]
  def -(that: Vector[T])(implicit ev: Numeric[T]): Vector[T]
  def *(that: Vector[T])(implicit ev: Numeric[T]): Vector[T]
  def /(that: Vector[T])(implicit ev: Fractional[T]): Vector[T]
}

trait Vector[T] {
  def toList(): List[T]
  def zeros(length: Int)(implicit ev: Numeric[T]): Vector[T]
}


object Vector {
  def apply[T](args: T*): Vector[T] = new VectorImpl[T](args.toList)

  private class VectorImpl[@specialized(Double, Int, Float, Long)T](val _data: List[T])
    extends Vector[T]
    with BinOps[T] {
    def +(that: Vector[T])(implicit ev: Numeric[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 + elem._2))
    def -(that: Vector[T])(implicit ev: Numeric[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 - elem._2))
    def *(that: Vector[T])(implicit ev: Numeric[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 * elem._2))
    def /(that: Vector[T])(implicit ev: Fractional[T]): Vector[T] = new VectorImpl[T](_data.zip(that).map(elem => elem._1 / elem._2))

    def fill(length: Int, value: T): Vector[T] = new VectorImpl[T](List.fill[T](length)(value))
    def zeros(length: Int)(implicit ev: Numeric[T]): Vector[T] = fill(length, ev.zero)
    def toList(): List[T] = _data.toList
  }

  implicit def VectorToList[T](v: Vector[T]): List[T] = v.toList
}