Implicit function types in Scala

Question

Recently, I have become interested in implicit functions. In the documentation, we can see several examples of using this property, but I think I do not quite understand how it works.

For example, we can read that implicit T0 => R is actually

trait ImplicitFunction1[-T0, R] extends Function1[T0, R] {
  override def apply(implicit x: T0): R
}

After writing the function below

val func = {implicit x: Int => 2*x}

I tried to use it in this way

implicit val x: Int = 3
println(func)

But it doesn't seem to work (only the <function1> type is returned, it looks like apply hasn't been used at all). If I had a method for it, it would work fine

def func(implicit x: Int) = 2*x

I'm not sure what I'm missing here.

Dmytro Mitin · Accepted Answer · 2019-09-01 18:56:54Z

5

Implicit function types work in Dotty

https://dotty.epfl.ch/docs/reference/contextual/implicit-function-types.html

https://dotty.epfl.ch/blog/2016/12/05/implicit-function-types.html

In Scala 2 func has type Int => Int instead of absent implicit Int => Int (aka given Int => Int).

implicit x: Int => ??? is just a shorthand for x: Int => { implicit val _x: Int = x; ???}.

Out of all new implicit (aka given) features in Dotty only by-name implicits were backported to Scala 2.13.0.

edited Sep 1, 2019 at 18:56

answered Sep 1, 2019 at 18:38

Dmytro Mitin

53.1k3 gold badges34 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

tribbloid Over a year ago

Wow, by-name implicits were backported to scala 2.13? So there is no need for Lazy type in shapeless? Saw the status is still 'PENDING' docs.scala-lang.org/sips/byname-implicits.html

Dmytro Mitin Over a year ago

@tribbloid By-name implicits were added in 2.13.0 github.com/scala/scala/releases/tag/v2.13.0 By-name implicits and shapeless.Lazy are not equivalent. I guess I met cases when one helped while the other didn't. Also you can write def foo(implicit tc: Lazy[TC[A]]): tc.value.Out = ??? but you can't write def foo(implicit tc: => TC[A]): tc.Out = ???. So Lazy is still necessary.

Collectives™ on Stack Overflow

Implicit function types in Scala

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related