Functional Interfaces in Scala-CodePudding

I have been working with Java for a while and now I am trying to learn Scala. I have a hard time understanding how to create a Functional Interface in Scala. I'm trying following code, but it's not working:

object Ex3 extends App {

trait Printer {
  def print(s: String): Unit
}

val p: Printer = x => println(x)

p("Hello") //does not want compile, error: 'Ex3.p.type' does not take parameters
}

p.s. I saw this example in an online course, and it worked.

UPD: I made a mistake in my code. It will work like this:

p.print("Hello)

also it works with apply method, as Mateusz Kubuszok said. Thanks everyone!

CodePudding user response：

val p: Printer = x => println(x) should indeed compile because Printer is a Single Abstract Method - its only abstract method is of signature String => Unit so if the compiler don't have to infer Printer, it can take the function that matches the method signature and add the rest.

But p("Hello") cannot work. To be able to call Printer value, it would have to have a method named apply defined, and there isn't one in your definition.

CodePudding user response：

We have this trait:

trait Printer {
  def print(s: String): Unit 
}

Now we can see, what happened if we try to implement it:

val p: Printer = new Printer {
  override def print(s: String): Unit = println(s)
}

We can see that our print method accepts one argument with the type String, so let's start with the function that accepts it:

val p: Printer = (s: String) => ???

And now add the implementation, we would like just print it, so let's do it:

val p: Printer = (s: String) => println(s)