I am trying to run a function in scala
def sum(xs: List[Int]): Int = xs match {
case Nil => throw new java.util.NoSuchElementException("Minimum number of elements")
case x :: xs => x + sum(xs)
}
When I try to run like this,
sum(List(1,2,3))
I am getting the runtime exception
java.util.NoSuchElementException: Minimum number of elements
at .sum(<console>:12)
at .sum(<console>:13)
On the other hand, this works
def sum(xs: List[Int]): Int = xs match {
case Nil => 0
case x :: xs => x + sum(xs)
}
List(1,2,3) is equivalent to 1::2::3::Nil
so your function is evaluated in following order
sum(1::2::3::Nil) = 1 + sum(2::3::Nil)
sum(2::3::Nil) = 2 + sum(3::Nil)
sum(3::Nil) = 3 + sum(Nil)
and at last sum(Nil) throws exception.
You can get more information from following question.
Why is Nil required at the end of a list built using the cons operator