Scala：如何将隐式参数应用于不同的范围？

Question

I can't find out why my implicits aren't in the scope. Here is a test:

package test

object Main extends App {
  import fp._

  def foo(a: Int)(implicit b: B0) = a + b.x
  print(foo(1))
}

Components in diff package:

package fp

trait Components {
  implicit val b0: B0 = B0(2)
}

object Components extends Components

sealed trait B
case class B0(x: Int) extends B

The error I got

could not find implicit value for parameter b: fp.B0
  print(foo(1))
           ^

Any ideas how to fix it?

Answer 1

You need to either do

import fp.Components._

or define the implicit value b0 in an implicit scope such as inside the companion object of B0 or the package object.

Here are some reading materials:

• https://www.artima.com/pins1ed/implicit-conversions-and-parameters.html
• http://docs.scala-lang.org/tutorials/FAQ/finding-implicits.html
• http://eed3si9n.com/implicit-parameter-precedence-again