Add "call-by-need" parameters evaluation

[riiswa]

Consider the following function:

def f(x: Int, lazy y: Int): Int = 
  if (x%2==0) x else y+y

With the lazy parameters, if we pass in parameter y the following block: {println("Hello World"); 1}, it will be evaluated 0 or 1 time, unlike the call-by-value which evaluates 1 time and the call-by-name which evaluates 0 or 2 times.

This behavior is possible in Scala in the following way:

def f(x: Int, lazy y: => Int): Int ={ 
  lazy val y_ = y
  if (x%2==0) x else y_+y_
}

I think it would be useful to have in Scala 3 the keyword lazy for function parameters.

[som-snytt]

scala/bug#240

[som-snytt]

or lampepfl/dotty-feature-requests#11