Question

Reading http://www.scala-lang.org/docu/files/ScalaByExample.pdf

This piece of code :

def While (p: => Boolean) (s: => Unit) {
  if (p) { s ; While(p)(s) }
}

Is given this explanation :

The While function takes as first parameter a test function, which
takes no parameters and yields a boolean value. As second parameter it
takes a command function which also takes no parameters and yields a
result of type Unit. While invokes the command function as long as the
test function yields true.

Where is if (p) evaluated to true or false ?

Should the function s not be declared somewhere ? There is no code for function s ?

Answer 1

Where is if (p) evaluated to true or false ?

Exactly there, in that line.

p and s are call-by-name parameters, because of the => in front of them in the parameter lists of the method While. Every time their name is used in the body of While, they are evaluated.

Should the function s not be declared somewhere ? There is no code for function s ?

s is a parameter to the While method, just like p. (Why are you asking this question about s, but not about p?). Methods and functions in Scala can have multiple parameter lists. The While method has two parameter lists.

You call this While method by passing it something that evaluates to Boolean (the parameter p), and a block (the parameter s).

var i = 0
While (i < 5) {
  i = i + 1
  println(i)
}

In this example p is i < 5, a function that evaluates to a Boolean, and s is the block between the { and }.