CODEWITHSUNDEEP
scalaimplicit
蘇哲聖
蘇哲聖

Reputation: 795

Scala implicit conversion conflict strangely

I found a strange compiling error:

class A
class B

object M {
  implicit val mA: M[A] = d => new A
  implicit val mB: M[B] = d => new B
}

trait M[K] {
  def get(d: D): K
}

object D {
  implicit def f[K](d: D)(implicit m: M[K]): K = m.get(d)
}

class D

object Main {
  val d = new D
  val a: A = d // This line can't compile!
}

That can't compile caused by mA and mB conflict. But that is strange since type B doesn't match our result type.

If I comment the mB like this

class A
class B

object M {
  implicit val mA: M[A] = d => new A
  //implicit val mB: M[B] = d => new B
}

trait M[K] {
  def get(d: D): K
}

object D {
  implicit def f[K](d: D)(implicit m: M[K]): K = m.get(d)
}

class D

object Main {
  val d = new D
  val a: A = d // can compile
}

That can compile. Why can't the first case compile?

Upvotes: 0

Views: 96

Answers (1)

talex
talex

Reputation: 20436

This happens because when compiler try to find implicit variable to pass as m to f it doesn't know expected return type (because it depends on which implicit variable will be selected). It creates kind of circular dependency.

PS: You don't need to make f implicit to illustrate this problem.

Upvotes: 3

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