Functions are reusable pieces of code, often called Blocks, that as the name suggests, act as building blocks to piece together a program.

Each language has its own keyword for defining a function block.

What is fun?

The fun keyword in Kotlin denotes a function, or method to wrap a reusable code block.

fun prepends the name of a function as illustrated below.

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fun functionName(): String {
    return "A String"
}

The structure of a Kotlin function

The function can take any number of parameters, and a return type.

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fun nameOfFunction(variable: InputType, anotherVariable: SomeType): ReturnType {
    // function code goes in here
    return somethingThatMatchesTheReturnType
}

A Kotlin function without a return value

If the function does not return a value, you can explicitly set its return type to Unit.

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fun doesNotReturnAnything(yourName: String): Unit {
  println("Hello "+yourName)
}