Implicit parameters of anonymous functions in Scala

Question

I'm a bit stuck with implicit parameters of anonymous functions. Hope somebody'll point me the right direction. Here is what I have. Two files: Main.scala and Foo.scala:

// Foo.scala
trait Fun[-A, +B] extends (A => B)

trait ImplicitString[+B] {
  def withString(block: String => B)(implicit s: String): B = block(s)
}

object FooFun extends Fun[String, String] with ImplicitString[String] {
  def apply(x: String): String = withString { implicit s =>
    x + s
  }
}

And

// Main.scala
object Main extends App {
  implicit val s = "it works!"
  println(FooFun("Test:"))
}

I'm expecting to see Test: it works! printed. But I got a compilation error:

$ scalac Main.scala Service.scala
Service.scala:8: error: could not find implicit value for parameter s: String
  def apply(x: String): String = withString { implicit s =>
                                        ^
one error found

Am I missing something?

UPDATE:

Looks like I should have imported my implicit val like this:

// Foo.scala
import Main._
...

This works fine.

(A => B) is nothing but Function[-A, +B], so your trait Fun really isn't needed. — Randall Schulz
– Randall Schulz, Commented Apr 9, 2014 at 14:45

tiran · Accepted Answer · 2014-04-09 12:45:29Z

1

If you are going to use s directly inside function, there's no point of marking it as implicit.

You can fix this in this way,

object FooFun extends Fun[String, String] with ImplicitString[String] {
  def apply(x: String)(implicit s1:String): String = withString { s =>
    x + s
  }
}

The problem is, implicit val s is not visible to the apply method body of FooFun.

answered Apr 9, 2014 at 12:45

tiran

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2 Comments

Vladimir Kostyukov Over a year ago

I tried this. This can't be compiled since FooFun is no longer implements Fun trait: there is no apply: String => String function in FooFun. There is just: apply: String => (String => String).

Vladimir Kostyukov Over a year ago

Folks, please, try to compile it before +1ing.

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