Scala Essentials: 隱式轉換

Class Extension

"+9519760513".exists(_.isDigit)

java.lang.String並存在exists方法，爲此標準庫中在Predef定義了一個隱式轉換，使String隱式地轉換爲StringOps，從而提供更多地操做字符串的方法。

object Predef {
  implicit def augmentString(x: String): StringOps = new StringOps(x)
}

Implicit Resolution Rules

Marking Rule

Only definitions marked implicit are available.

object Predef {
  implicit def intWrapper(x: Int) = new scala.runtime.RichInt(x)
}

object Predef {
  implicit final class any2stringadd[A](private val self: A) extends AnyVal {
    def +(other: String): String = String.valueOf(self) + other
  }
}

Scope Rule

An inserted implicit conversion must be in scope as a single identifier, or be associated with the source or target type of the conversion.

case class Yard(val amount: Int)
case class Mile(val amount: Int)

mile2yard能夠定義在object Mile

object Mile {
  implicit def mile2yard(mile: Mile) = new Yard(10*mile.amount)
}

也能夠定義在object Yard中

object Yard {
  implicit def mile2yard(mile: Mile) = new Yard(10*mile.amount)
}

轉換爲目標類型時，經常發生以下兩個場景：

• 傳遞參數時，但類型匹配失敗；

def accept(yard: Yard) = println(yard.amount + " yards")
accept(Mile(10))

• 賦值表達式，但類型匹配失敗

val yard: Yard = Mile(10)

Other Rules

• One-at-a-time Rule: Only one implicit is tried.

• Explicits-First Rule: Whenever code type checks as it is written, no implicits are attempted.

• No-Ambiguty Rule: An implicit conversion is only inserted if there is no other possible conversion is inserted.

Where implicits are tried?

• Conversions to an expected type

  • 傳遞參數時，但類型匹配失敗；

  • 賦值表達式，但類型匹配失敗

• Conversions of the receiver of a selection

  • 調用方法，方法不存在

  • 調用方法，方法存在，但參數類型匹配失敗

• Implicit parameters

Implicit parameters

import scala.math.Ordering

case class Pair[T](first: T, second: T){
  def smaller(implicit order: Ordering[T]) =
    order.min(first, second)
}

當T爲Int

Pair(1, 2).smaller

編譯器實際調用：

Pair(1, 2).smaller(Ordering.Int)

其中Ordering.Int定義在Ordering的伴生對象中

object Ordering {
  trait IntOrdering extends Ordering[Int] {
    def compare(x: Int, y: Int) =
      if (x < y) -1
      else if (x == y) 0
      else 1
  }
  implicit object Int extends IntOrdering
}

也就是說

implicitly[Ordering[Int]] == Ordering.Int  // true

其中，implicitly爲定義在Predef的一個工具函數，用於提取隱式值

@inline def implicitly[T](implicit e: T) = e

當T爲自定義類型

import scala.math.Ordering

case class Point(x: Int, y: Int)

object Point {
  implicit object OrderingPoint extends Ordering[Point] {
    def compare(lhs: Point, rhs: Point): Int =
      (lhs.x + lhs.y) - (rhs.x + rhs.y)
  }
}

Pair(Point(0, 0), Point(1, 1)).smaller

等價於

Pair(Point(0, 0), Point(1, 1)).smaller(Point.OrderingPoint)

也就是說

implicitly[Ordering[Point]] == Point.OrderingPoint

Context Bound

import scala.math.Ordering

case class Pair[T : Ordering](first: T, second: T) {
  def smaller(implicit order: Ordering[T]) = order.min(first, second)
}

能夠使用implicitly簡化

import scala.math.Ordering
case class Pair[T : Ordering](first: T, second: T) {
  def smaller = implicitly[Ordering[T]].min(first, second)
}

能夠進一步簡化

import scala.math.Ordering

case class Pair[T : Ordering](first: T, second: T) {
  def smaller = Ordering[T].min(first, second)
}

Ordering[T]首先調用了object Ordering的apply方法，從而便捷地找到了Order[T]的隱式值

object Ordering {
  def apply[T](implicit ord: Ordering[T]) = ord
}

因此Ordering[T].min等價於implicitly[Ordering[T]].min

View Bound

import scala.math.Ordered

case class Pair[T](first: T, second: T){
  def smaller(implicit order: T => Ordered[T]) = {
    if (order(first) < second) first else second
  }
}

implicit order: T => Ordered[T]在smaller的局部做用域內，便是一個隱式參數，又是一個隱式轉換函數。

import scala.math.Ordered

case class Pair[T](first: T, second: T){
  def smaller(implicit order: T => Ordered[T]) = {
    if (first < second) first else second
  }
}

又由於在Predef預約義了從Int到RichInt的隱式轉換，而RichInt是Ordered[Int]的子類型，因此在Predef定義的implicit Int => RichInt的隱式轉換函數可做爲隱式參數implicit order: T => Ordered[T]的隱式值。

Pair(1, 2).smaller

等價於

Pair(1, 2).smaller(Predef.intWrapper _)

上述簡化的設計，使得隱式參數order沒有必要存在，這樣的模式較爲常見，可歸一爲通常模式：View Bound

import scala.math.Ordered

case class Pair[T <% Ordered[T]](first: T, second: T) {
  def smaller = if (first < second) first else second
}

須要注意的是：T <% Ordered[T]表示：T能夠隱式轉換爲Ordered[T]；而T <: Ordered[T]表示：T是Ordered[T]的一個子類型。

Upper Bound

import scala.math.Ordered

case class Pair[T <: Comparable[T]](first: T, second: T) {
  def smaller = if (first.compareTo(second) < 0) first else second
}

Pair("1", "2").smaller  // OK, String is subtype of Comparable[String]
Pair(1, 2).smaller      // Compile Error, Int is not subtype of Comparable[Int]