Pattern matching on a list in Scala

I think the following should work:

def flatten(l: List[_]): List[Any] = l match {
  case Nil => Nil
  case (head: List[_]) :: tail => flatten(head) ::: flatten(tail)
  case head :: tail => head :: flatten(tail)
}

The first line is a match for Nil, so if we don't find anything return nothing. The second line will identify List of Lists and recall the flatten method and flatten the list of lists.

I would also like to mention that the case for lists can be divided not only the head and tail, as well as any N number of list elements:

def anyFunction(list: List[Int]): Unit =
  list match {
        // ...methods that have already been shown
      case first :: second :: Nil  => println(s"List has only 2 elements: $first and $second")
      case first :: second :: tail => println(s"First: $first \nSecond: $second \nTail: $tail")
  }

Hope it will be useful to someone.

Unless you are repeatedly calling simple_fun in some way, what you have there will pattern match the first element and nothing more. To get it to match the whole list, you can get simple_fun to call itself recursively, like this:

val simplelist: List[Char] = List('a', 'b', 'c', 'd')

def simple_fun(list: List[Char]): List[Nothing] = list match {
  case x :: xs => {
    println(x)
    simple_fun(xs)
  }
  case _ => Nil 
}

Note I've also left out some of the types as the Scala compiler can infer them, leaving you with less cluttered, more readable code.

As a small side-note, calling println repeatedly inside the function like that is not particularly functional - as it is all about side effects. A more idiomatic approach would be to have the function construct a string describing the list, which is then output with a single call to println - so the side-effects are kept in a single well-defined place. Something like this would be one approach:

def simple_fun(list: List[Char]):String = list match {
  case x :: xs => x.toString + simple_fun(xs)
  case Nil => ""
}

println(simple_fun(simple_list))