Type inference

When we call a function with an anonymous function for f, we have to specify the type of its argument, here named x:

Typed function literal

def filter[A](l: List[A], f: A => Boolean): List[A] = l.filter(f)
// filter: [A](l: List[A], f: A => Boolean)List[A]

filter(List(-3, -2, -1, 0, 1, 2, 3), (x: Int) => x > 0)
// res1: List[Int] = List(1, 2, 3)

filter(List(-3, -2, -1, 0, 1, 2, 3), x => x > 0)
// <console>:13: error: missing parameter type
//        filter(List(-3, -2, -1, 0, 1, 2, 3), x => x > 0)
//                                             ^

When a function definition contains multiple argument groups (in other words, curried), type information flows from left to right across these argument groups. The main reason for grouping the arguments this way is to assist with type inference.

Curried version

filter(List(-3, -2, -1, 0, 1, 2, 3))(x => x > 0)
res7: List[Int] = List(1, 2, 3)