Example of grouping list items based on custom predicate

GroupList.scala

val li=List(2, 3, 4, 5, 6, 10, 11,20, 21, 22)     //> li  : List[Int] = List(2, 3, 4, 5, 6, 10, 11, 20, 21, 22)
                       
  def groupList[T](list: List[T],inGroup: (T,T)=>Boolean) = {
    def group(lst: List[T], acc: List[T]): List[List[T]] = (lst,acc) match {
      case (Nil,_) => acc.reverse :: Nil
      case (h :: t,Nil) => group(t, h :: acc)
      case (h :: t,ah :: _) if inGroup(h,ah) => group(t, h :: acc)
      case (h :: t,_)                       => acc.reverse :: group(t, h :: Nil)
    }
    group(list, List.empty)
  }                                               //> groupList: [T](list: List[T], inGroup: (T, T) => Boolean)List[List[T]]

def oneApart(x:Int,y:Int)=x-y==1                  //> oneApart: (x: Int, y: Int)Boolean

groupList(li,oneApart)                            //> res0: List[List[Int]] = List(List(2, 3, 4, 5, 6), List(10, 11), List(20, 21,
                                                  //|  22))

def fourOrLessApart(x:Int,y:Int)=x-y <=4          //> fourApart: (x: Int, y: Int)Boolean

groupList(li,fourOrLessApart)                     //> res1: List[List[Int]] = List(List(2, 3, 4, 5, 6, 10, 11), List(20, 21, 22))
                                                  //| 

val li2="Hello wooorld".toList                    //> li2  : List[Char] = List(H, e, l, l, o,  , w, o, o, o, r, l, d)

def same(x:Char,y:Char)=x==y                      //> same: (x: Char, y: Char)Boolean

groupList(li2,same)                               //> res2: List[List[Char]] = List(List(H), List(e), List(l, l), List(o), List( )
                                                  //| , List(w), List(o, o, o), List(r), List(l), List(d))