CODEWITHSUNDEEP
scalatypeclass
Rob
Rob

Reputation: 3499

How does scala handle implicit typeclasses without instantiating a member?

I notice when I'm trying to create an instance of a fooSemigroup, in version one the anonymous function creates an instance of fooSemigroup without instantiating a member of Foo, but when I try to do this without the SAM construct, I need to create a dummy instance of foo for the SemigroupOps to pull in the implicit value ev.

trait Semigroup[A] {
  def combine(x: A, y: A): A
}
case class Foo(v: Int)

// vs 1
implicit val fooSemigroup: Semigroup[Foo] = (x: Foo, y: Foo) => Foo(x.v + y.v)

// vs 2
class fooSemigroup(foo: Foo) extends Semigroup[Foo] {
  def combine(x: Foo, y: Foo) = Foo(x.v + y.v)
}
implicit val f: fooSemigroup = Foo(0)

implicit class SemigroupOps[A](x: A) {
  def +(y: A)(implicit ev: Semigroup[A]): A = ev.combine(x, y)
}

Upvotes: 0

Views: 67

Answers (1)

Mario Galic
Mario Galic

Reputation: 48430

Instead of implicit class

implicit class fooSemigroup(foo: Foo) extends Semigroup[Foo] {
  def combine(x: Foo, y: Foo) = Foo(x.v + y.v)
}

try implicit object

implicit object fooSemigroup extends Semigroup[Foo] {
  def combine(x: Foo, y: Foo) = Foo(x.v + y.v)
}

However as per Luis' advice favour implicit val instances over implicit object as otherwise resolution might result in ambiguous implicit values error, for example

trait SomeTrait[T] {
  def f: T
}
trait ExtendedTrait[T] extends SomeTrait[T] {
  def g: T
}
implicit object SomeStringObject extends SomeTrait[String] {
  override def f: String = "from object f"
}
implicit object ExtendedStringObject extends ExtendedTrait[String] {
  override def f: String = "from extended obj f"
  override def g: String = "from extended obj g"
}

implicitly[SomeTrait[String]] // Error: ambiguous implicit values

where full error states

Error:(15, 77) ambiguous implicit values:
 both object ExtendedStringObject in class A$A7 of type A$A7.this.ExtendedStringObject.type
 and object SomeStringObject in class A$A7 of type A$A7.this.SomeStringObject.type
 match expected type A$A7.this.SomeTrait[String]
def get$$instance$$res0 = /* ###worksheet### generated $$end$$ */ implicitly[SomeTrait[String]];}
                                                                            ^

Upvotes: 4

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