Question

In Scala, does calling isEmtpy method on an instance of Stream class cause the stream to be evaluated completely? My code is like this:

import Stream.cons

private val odds: Stream[Int] = cons(3, odds.map(_ + 2))
private val primes: Stream[Int] = cons(2, odds filter isPrime)

private def isPrime(n: Int): Boolean = n match {
  case 1 => false
  case 2 => true
  case 3 => true
  case 5 => true
  case 7 => true
  case x if n % 3 == 0 => false
  case x if n % 5 == 0 => false
  case x if n % 7 == 0 => false
  case x if (x + 1) % 6 == 0 || (x - 1) % 6 == 0 => true
  case x => primeDivisors(x) isEmpty
}


import Math.{sqrt, ceil}
private def primeDivisors(n: Int) =
  primes takeWhile { _ <= ceil(sqrt(n))} filter {n % _ == 0 }

So, does the call to isEmpty on the line case x => primeDivisors(x) isEmpty cause all the prime divisors to be evaluated or only the first one?

Answer 1

Only if the stream is actually empty 🙂

Otherwise, it will just see if the stream has a head and tail (matches Stream.cons) and return false.