Question

The following program, was compiled and tested, it sometimes return the result, and sometimes fills the screen with

java.lang.StackOverflowError
at scala.BigInt$.apply(BigInt.scala:47)
at scala.BigInt.equals(BigInt.scala:129)
at scala.runtime.BoxesRunTime.equals(Unknown Source)
at bigint$.factorial(fact2.scala:3)
at bigint$.factorial(fact2.scala:3)
...

The program:

object bigint extends Application {
  def factorial(n: BigInt): BigInt = if (n == 0) 1 else n * factorial(n-1)
  println("4391! = "+factorial(4391))
}

My questions:

• Is it because there is a stack overflow on the JVM, which sometimes happens and sometimes doesn’t?
• Is this nondeterministic behavior considered a bug?
• I assume Scala did not tail-recursed this? how can I make it tail-recurse this?

Details:

Scala compiler version 2.7.5.final —
Copyright 2002-2009, LAMP/EPFL Scala
code runner version 2.7.5.final —
Copyright 2002-2009, LAMP/EPFL

java version “1.6.0_0” OpenJDK
Runtime Environment (build
1.6.0_0-b11) OpenJDK Client VM (build 1.6.0_0-b11, mixed mode, sharing)

Ubuntu 2.6.24-24-generic

Answer 1

Tail-call optimization will only work in Scala if the recursive call is the last statement in the function. It’s very limited. The Scala book says:

[…] tail-call optimization is
limited to situations in which a
method or nested function calls itself
directly as its last operation,
without going through a function value
or some other intermediary.

In your case, the recursive call is part of a larger expression, and is not itself the very last operation – the last operation here is the multiplication.

This article demonstrates how to make it work:

class Factorial {
  def factorial(n: Int): Int = {
    def factorialAcc(acc: Int, n: Int): Int = {
      if (n <= 1) acc
      else factorialAcc(n * acc, n - 1)
    }
    factorialAcc(1, n)
  }
}