12 Anonymous Functions

Anonymous functions in Scala are of the following forms:

(x: Int) => x * x //type is: Int => Int, e.g. gets an Int and returns an Int
(x: Int, y: Int) => x + y //type is: (Int, Int) => Int, e.g. gets 2 Ints and returns an Int

Which is basically a “syntactic sugar” for this form:

new Function1[Int ,Int] {
  def apply(x: Int): Int = x * x

new Function2[Int ,Int ,Int] {
  def apply(x: Int, y: Int): Int = x + y


The type of the 2nd anonymous function is (Int, Int) => Int and reads: “A function that map from two integers (Int, Int) to (=>) an integer (Int)”

The method doWithOneAndTwo expects a parameter of that type, so we can pass (x ,y) => x + y as a parameter.

Parameter type inference

Note that we were able to drop the type declarations for x and y here, becase the compiler already “knows” that doWithOneAndTwo expects a function that gets 2 Int parameters, therefore we can omit the type information for the parameters x and y in the second call in the example on the left.

Shorter syntax

Furthermore, there is even a shorter syntax for anonymous functions, (with a limitation that each variable is used exactly once in the body of the function). A place holder is used _ to replace each parameter, the first occurrence of _ stands for the first parameter, the second for the second parameter and so forth.

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