Getting "error: type mismatch; found : Unit required: () => Unit" on callback

Question

I am just starting out going through a tutorial on scala and have hit a block. I have merged together a couple of examples and am getting an error, but don't know why.

import java.text.DateFormat._
import java.util.{Date, Locale}

object FrenchDate {
  def main(args: Array[String]) {
    timer(println(frenchDate))
  }

  def frenchDate():String = {
    val now = new Date
    val df = getDateInstance(LONG, Locale.FRANCE)
    df format now
  }

  def timer(callback: () => Unit) {
    while(true) {callback(); Thread sleep 1000}
  }
}

Brings the error

error: type mismatch;
found   : Unit
required: () => Unit
println(frenchDate)

while the below works

import java.text.DateFormat._
import java.util.{Date, Locale}

object FrenchDate {
  def main(args: Array[String]) {
    timer(frenchDate)
  }

  def frenchDate() {
    val now = new Date
    val df = getDateInstance(LONG, Locale.FRANCE)
    println(df format now)
  }

  def timer(callback: () => Unit) {
    while(true) {callback(); Thread sleep 1000}
  }
}

The only difference is that the date is printed out in frenchDate() in the second once whereas it is returned and printed in the callback on the first.

Answer 1

The difference is that this line:

timer(println(frenchDate))

is trying to call println(frenchDate) and use the return value (which is Unit) as the callback to pass to timer. You probably want:

timer(() => println(frenchDate))

or possibly

timer(() => { println(frenchDate) })

(I'm not a Scala dev, so I'm not sure of the right syntax, but I'm pretty confident about what's wrong in your current code :)

EDIT: According to comments, this should work too and may be more idiomatic:

timer { () => println(frenchDate) }