Scala 隐式和类型别名

Question

假设我有以下代码:

object Potato extends App {

  type CoolString = String
  type AwesomeString = String

  def potato(string: String)(implicit coolString: CoolString, awesomeString: AwesomeString) = {
    s"$string is a string. Oh, but don't forget - $coolString and also $awesomeString"
  }

  implicit val ice : CoolString = "Really Cold Ice"
  implicit val awe : AwesomeString = "Awe inspiring object"

  potato("Stringerino")

}

此代码因问题而失败

[error] ... ambiguous implicit values:
[error]  both value ice in object Potato of type => Potato.CoolString
[error]  and value awe in object Potato of type => Potato.AwesomeString
[error]  match expected type Potato.CoolString
[error]   potato("Stringerino")

这样使用隐式是不可能的吗？

Answer 1

Is such use of implicits impossible?

依赖这样一种方法，将像 String 这样的通用类型作为隐式传入，不仅是不可能的，而且是危险的。想想看，范围内的任何 String 实例都将是传递到该方法的合格候选者!

类型别名只是类型别名，仅此而已。对于编译器来说，CoolString 和 AwesomeString 都只是 String。

更好的方法是利用标记类型。例如，这是 tagged type using shapeless :

import shapeless.tag.@@

trait CoolString
trait AwesomeString

type ReallyCoolString = String @@ CoolString
type ReallyAwesomeString = String @@ AwesomeString

然后:

import shapeless.tag

def potato(string: String)(implicit coolString: ReallyCoolString, awesomeString: ReallyAwesomeString) = {
  s"$string is a string. Oh, but don't forget - $coolString and also $awesomeString"
}

def main(args: Array[String]): Unit = {
  implicit val ice = tag[CoolString][String]("Really Cold Ice")
  implicit val awe = tag[AwesomeString][String]("Awe inspiring object")

  println(potato("Stringerino"))
}

产量:

Stringerino is a string. 
Oh, but don't forget - Really Cold Ice and also Awe inspiring object