Scala: An extension method was tried, but could not be fully constructed (same extension name on multiple classes)

user_1204440 0
Viewed 188

In Scala, I'm trying to define an extension method on two different classes. I want the extension method to have the same name, for both classes.

Note that the classes aren't related by inheritance.

As an example, say I have two case classes A and B, each with one String property, and I want to define a lower method on both classes, which will return the lowercase of their property:

import SomeObject.AExt.lower
import SomeObject.BExt.lower

object SomeObject {

  case class A(foo: String)

  case class B(bar: String)

  object AExt {
    extension (x: A) def lower = x.foo.toLowerCase
  }

  object BExt {
    extension (x: B) def lower = x.bar.toLowerCase
  }

  def hello(): Unit = {
    def a = A("foo")

    def b = B("bar")

    println(a.lower)
    println(b.lower)
  }
}

The compilation fails with this error:

An extension method was tried, but could not be fully constructed:

    SomeObject.BExt.lower(a)    failed with

        Found:    SomeObject.A
        Required: SomeObject.B

Looks like the compiler is confused, and is trying to apply the lower extension defined on class B, to my instance of A.

scala-3 extension-methods scala
1 Answers

One workaround is to put the extension methods inside the same object:

import SomeObject.ABExt.lower
object SomeObject {

  case class A(foo: String)

  case class B(bar: String)

  object ABExt {
    extension (x: A) def lower = x.foo.toLowerCase
    extension (x: B) def lower = x.bar.toLowerCase
  }

  def hello(): Unit = {
    def a = A("foo")

    def b = B("bar")

    println(a.lower)
    println(b.lower)
  }
}
user_12044400
Related