Why is the implicit conversion not considered in this case with generic parameters?

问题

Consider the following code, derived from the metascala project:

object Units {
  case class Quantity[M <: MInt, T: Numeric](value: T) {
    type This = Quantity[M, T]

    def *[M2 <: MInt](m: Quantity[M2, T]) = 
      Quantity[M + M2, T](numeric[T].times(value, m.value))
    def /[M2 <: MInt](m: Quantity[M2, T]) = 
      Quantity[M - M2, T](numeric[T].div(value, m.value))
    def apply(v: T) = Quantity[M, T](numeric[T].times(v, value))
  }

  implicit def measure[T: Numeric](v: T) = Quantity[_0, T](v)

  implicit def numericToQuantity[T: Numeric](v: T) = 
    new QuantityConstructor[T](v)

  class QuantityConstructor[T: Numeric](v: T) {
    def m = Quantity[_1, T](v)
  }
}

(the MInt is basically the implementation of peano numbers where _0, _1 are concrete "values" from metascala. Tell me if you need additional code, I just didn't want to paste everything in here.)

I want to support code where some existing Quantity can be multiplied by a simple number, e. g.

import Units._

val length1 = 5  * (5 m) //doesn't work <-----
val length2 = (5 m) * 5  // works

Why is the implicit method measure not considered in the first line of code as I have assumed?

Instead I get this error message:

overloaded method value * with alternatives:
   (x: Double)Double <and>
   (x: Float)Float <and>
   (x: Long)Long <and>
   (x: Int)Int <and>
   (x: Char)Int <and>
   (x: Short)Int <and>
   (x: Byte)Int  cannot be applied to 
(scalax.units.Units3.Quantity[scalax.units.Integers._1,Int])

I'm using Scala 2.10-trunk.

This is actually the follow up to How does ‘1 * BigInt(1)’ work and how can I do the same?.

回答1:

To get things started, here's a stand-alone example that gives the same problem,

object Units {
  case class Quantity[T: Numeric](value: T) {
    def *[M](m: Quantity[T]) =           // type M can't be inferred below
      Quantity[T](implicitly[Numeric[T]].times(value, m.value))
  }
  implicit def measure[T: Numeric](v: T) = Quantity[T](v)

  val length0 = measure(5) * Quantity(5) // works
  val length1 = 5  * Quantity(5)         // doesn't work
}

For some reason, the conversion measure isn't being found because of the type parameter M on the method *. If the type parameter is removed from *, things compile fine. Maybe someone else can explain why?

Edit. This is looking like a limitation of the Scala compiler, since renaming * to something like *** resolves the problem. Perhaps the existence of Int.* (without a type parameter) is precluding the implicit conversion for use of Quantity.*[M] (with a type parameter). This reminds me of the requirement that overridden methods must have the same exact type parameters.

来源：https://stackoverflow.com/questions/7649517/why-is-the-implicit-conversion-not-considered-in-this-case-with-generic-paramete

标签

scala

implicit

units-of-measurement