从List [Int]到List [Double]的隐式转换失败

问题描述

我是Scala新手，但是我注意到一种我不了解的行为.当我执行此代码时，一切都会顺利进行:

I am a Scala newbie and I've noticed a behaviour I do not understand. When I execute this code everything goes just fine:

val lD: List[Double] = List(1, 2, 3)

但是，当我执行此命令时:

However when I execute this one:

val lI = List(1, 2, 3)
val lD: List[Double] = lI

我得到一个错误

<console>:11: error: type mismatch;
 found   : List[Int]
 required: List[Double]

能否请您提供一个线索，为什么第二个清单中没有进行隐式转换?

May you please give me a clue why an implicit conversion does not take place in the second listing?

推荐答案

没有定义从List [Int]到List [Double]的隐式转换，尽管如果您确实想要一个，也可以创建一个.请参阅以下内容:

There's no implicit conversion defined from List[Int] to List[Double], although if you really want one you can make one. See the following:

Welcome to Scala version 2.9.1.final (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_37).
Type in expressions to have them evaluated.
Type :help for more information.

scala> val i = 1
i: Int = 1

scala> val d: Double = i
d: Double = 1.0

scala> val il = List(1,2,3,4)
il: List[Int] = List(1, 2, 3, 4)

scala> val dl: List[Double] = il
<console>:8: error: type mismatch;
 found   : List[Int]
 required: List[Double]
       val dl: List[Double] = il
                              ^

scala> object il2dl { implicit def intlist2dlist(il: List[Int]): List[Double] = il.map(_.toDouble) }
defined module il2dl

scala> import il2dl._
import il2dl._

scala> val dl: List[Double] = il
dl: List[Double] = List(1.0, 2.0, 3.0, 4.0)

scala>

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