Partial application means calling a function with less arguments than it has and saving the result as another function (that waits for the rest of the arguments).
multiplyBy: Int -> Int -> Int multiplyBy x y = x * y multiplyByTwo : Int -> Int -- one Int has disappeared! we now know what x is. multiplyByTwo = multiplyBy 2 > multiplyByTwo 2 4 : Int > multiplyByTwo 4 8 : Int
As an academic sidenote, Elm can do this because of currying behind the scenes.