scala found Unit required Unit.type - scala

Question

I have the following:
object T {
abstract class First {
def doSomething= (s:String) => Unit
}
class Second extends First {
override def doSomething = {
(s:String) => ()
}
}
def main(args: Array[String]): Unit = {
new Second().doSomething
}
}
but this fails to compile with the error:
Error:(8, 21) type mismatch;
found : Unit
required: Unit.type
(s:String) => ()
Why isn't the override from class Second valid? How could I make it work?

Answer 1

The problem is that (s:String) => Unit is returning Unit.type - instead change it to (s:String) => () (or to a method if you didn't mean to be returning Function1[String, Unit] from your method.)
To put it another way:
def doSomething = (s:String) => Unit
is really:
def doSomething: (String) => Unit.type =
// A function that takes a string and returns the companion type of Unit
(s: String) => Unit
While what you probably wanted was:
def doSomething: (String) => Unit =
// A function that takes a string and returns Unit
(s: String) => ()
Or maybe:
def doSomething(s: String): Unit
// An abstract method that takes a string and returns nothing.

Answer 2

(s:String) => Unit returns Unit.type ie it returns the type rather than a value of that type. You want to do (s:String) => (), which will return a value, whose type is Unit.

Answer 3

' trait Processor0 {
def process = (x:String) => ()
}
case class Processor2() extends Processor0 {
override def process = (x:String) => println("Processor2 x :" + x)
}
case class Processor3() extends Processor0 {
override def process = (x:String) => println("Processor3 x :" + x)
}
object DemoProc2 {
def main( args : Array[String]):Unit ={
val s:String = "yes"
val myFuncs: Map[String,(String) => () ]= Map( //Error 1. '=>' expected but ']' found. 2.identifier expected but ';' found.
"string2" -> (() => ProcessorTwo().process(s)),
"string3" -> (() => ProcessorThree().process(s))
)
myFuncs.values.foreach(v => v());
}
} '