Functional Programming Principles in Scala week-6 Anagram code snippet

combination.scala

/**
  计算全部子集

  Input:  List( ('a', 2), ('b', 2))
  Output: 

Nil

List(('a', 1))  
List(('a', 2)) 

List(('b', 1))  
List(('b', 2)) 

List(('a', 1), ('b', 2))  
List(('a', 2), ('b', 2))  
List(('a', 1), ('b', 1))  
List(('a', 2), ('b', 1))

**/

type Occurrences = List[(Char, Int)]
def combination(occ: Occurrences): List[Occurrences] = occ match {
  case Nil =>
    List(List())
  case head :: tail =>
    for{i <- (0 to head._2).toList
        rest <- combination(tail)
    } yield ((head._1, i) :: rest).filter(_._2 != 0)
}