scala flatMap flatten nested lists

Question

I'm learning Scala and going through 99 Scala problems. For the following exercises:

Flatten a nested list structure. Example:

scala> flatten(List(List(1, 1), 2, List(3, List(5, 8))))
res0: List[Any] = List(1, 1, 2, 3, 5, 8)

The solution is given as

def flatten(ls: List[Any]): List[Any] = ls flatMap {
  case ms: List[_] => flatten(ms)
  case elements => List(elements)
}

But I was wondering why the following does not work?

def flatten[A](ls: List[List[A]]): List[A] = ls flatMap {
  case ms: List[_] => flatten(ms)
  case elements => List(elements)
}

IntellJ IDEA tells me the issue is with the flatten(ms) part, saying "cannot resolve reference flatten with such a signature" and in the List class documentation for flatten it says "Note: The compiler might not be able to infer the type parameter".

Any ideas why the second code does not work?

Rhys Bradbury Rhys Bradbury · Accepted Answer · 2016-05-25T10:05:12

Someone else had a different question, but using the same example.

I break down each step and explain it here

scala flatMap flatten nested lists

2 Answers