scala> val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val numbers: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

scala> val numberFunc = numbers.foldLeft(List[Int]())
val numberFunc: ((List[Int], Int) => List[Int]) => List[Int] = Lambda$1752/0x0000008001565820@a4dcede

scala> numberFunc((xs, x) => xs :+ x*x)
val res1: List[Int] = List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)

scala> val numberFunc = numbers.foldLeft(List[Int]()) _
1 warning found
-- Warning: ------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 |val numberFunc = numbers.foldLeft(List[Int]()) _
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                 The syntax `<function> _` is no longer supported;
  |                 you can simply leave out the trailing ` _`
val numberFunc: ((List[Int], Int) => List[Int]) => List[Int] = Lambda$1786/0x00000080015b59c0@2634d000