Definition

• Higher-order functions are the ones that take functions as parameters or return function as a result.
• We can even store the higher-order function in a variable and a data structure.
• They enable functional programming concepts and allow you to write more concise and expressive code.

Declaring Functions

• Functions in kotlin have a type
• The same way as we can declare Int,String,Person we can declare something of type function
• A type is composed of FunctionParameters + FunctionReturnType
• (Parameters are declared within these brackets) --> ReturnType

Examples of declaring functions

A function that takes a single parameter Int and returns a Unit(A unit means nothing like void)

(Int) -> Unit

A function that takes two parameters Int and String and returns Unit

(Int,String) -> Unit

A function that takes String as a parameter and returns Int

(String) -> Int

Passing Functions

• We can pass functions to other functions and return functions from others.
• How we do this
  • Declare a function as having another function as a parameter.
  • When using the first function, We pass the second function as an argument.

More reading

• kotlin literals
• Lamda Expression
• Passing a function as parameter
• Returning Function
• Higher Order Functions: Examples