3

Can some one explain me why scala resolve the most generic implicit no matter of local scope implicit more specific ?

Example:

import scala.math.ScalaNumber

type Serializer[T] = T => String

object SerializedOps{
  implicit class AnyOps[T](t: T){
    def serialize(implicit s: Serializer[T]) : String = s(t)
  }
}

object Instances{
  implicit val scalaNumber : Serializer[ScalaNumber] = _.toString + "_DEFAULT"
}


import SerializedOps._
import Instances._


implicit val bigDecimalCustom : Serializer[BigDecimal] = _.toString + "_CUSTOM"

val res: String = BigDecimal(100).serialize
//res: String = 100DEFAULT

Why i can't define a new implicit more specific in my local scope? How scala resolve implicits ?

Mario Galic
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David Geirola
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  • Take a look to the [**implicits priority guideline**](https://typelevel.org/cats/guidelines.html) - And [**where does scala look for implicits**](https://docs.scala-lang.org/tutorials/FAQ/finding-implicits.html). – Luis Miguel Mejía Suárez Jul 04 '19 at 18:21

1 Answers1

4

Subtype does win:

If there are several eligible arguments which match the implicit parameter's type, a most specific one will be chosen using the rules of static overloading resolution.

however function is contravariant over its argument type which makes 

ScalaNumber => String

a subtype of 

BigDecimal => String

thus ScalaNumber => String is more specific. Note that type Serializer[T] = T => String is aliasing a function type.

Mario Galic
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  • Now it’s clear, but there is a way to do what i need ? Define a generic Serializer and define a more specific one in local scope ? – David Geirola Jul 04 '19 at 18:46