Question

Is there any difference between the following

def foo(s: String) = { ... }

and

def foo(s: => String) { ... }

both these definitions accept “sss” as parameter.

Answer 1

An argument String is a by-value parameter, => String is a by-name parameter. In the first case, the string is passed in, in the second a so-called thunk which evaluates to a String whenever it is used.

def stringGen: String = util.Random.nextInt().toString

def byValue(s: String) =
  println("We have a '" + s + "' and a '" + s + "'")

def byName(s: => String) =
  println("We have a '" + s + "' and a '" + s + "'")

byValue(stringGen)  // constant value
byName (stringGen)  // evaluated twice

Often a by-name parameter is not used to evaluate it several times, but to lazily evaluate it once.

def logMessage = {
  println("Calculating log message...")
  new java.util.Date().toString
}

def log(enabled: Boolean, message: => String): Unit = {
  lazy val fullMessage = "LOG: " + message
  println("Test")
  if (enabled) println(fullMessage)
}

log(false, logMessage)
log(true , logMessage)