Question

I stumbed on the link below which shows how funny scala messages can be 😉

Scala REPL “She’s gone rogue” error message

When i tried this example, it hung forever.what is happening here?

Is this a known bug?Is it some kind of gotcha?

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).
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scala> def factorial(n: Int):Int = {
     |   if (n == 1) n    // forgot 'return' here
     |   factorial(n - 1)
     | }
factorial: (n: Int)Int

scala> factorial(10)

Answer 1

The problem is exactly what is indicated in the code, return is missing. The recursion never terminates.

factorial(n - 1) is not connected to the if statement in any way. This means, that it executes unconditionally. factorial() wil always call itself.

Adding a base case (i.e. where the recursion stops):

def factorial(n: Int):Int = {
     |   if (n == 1) return n
     |   factorial(n - 1)
     | }

This would work as well:

def factorial(n: Int):Int = {
     |   if (n == 1) n
     |   else factorial(n - 1)
     | }

This will actually produce the factorial of n (the others return 1):

def factorial(n: Int):Int = {
     |   if (n <= 1) 1
     |   else n * factorial(n - 1)
     | }

Note that this works for the case n = 0 correctly as well. It still accepts negative numbers, however.