Writing a Function

In this lesson you will learn the basic way to define a function in Kotlin so you can reuse the same logic over and over. This is for anyone searching for "Kotlin functions how to use".

You define a function with fun name(param: Type): ReturnType. Bundling frequently used work under a name means you only have to call it to run it.

The sample code has greet take a name: String parameter and return a greeting, with the return type stated explicitly as : String. Like a recipe, once you have named a set of steps you can reproduce the same result whenever you need it.

A common stumbling block for beginners is how the parameter and return types are written. Kotlin writes paramName: Type and puts the return type after the function name with a colon, and that distinctive ordering takes some getting used to.

In real-world development, gathering shared logic into functions reduces duplication and means a fix only has to be made in one place.

📖 Reference code

fun greet(name: String): String {
    return "Welcome, ${name}!"
}
fun main() {
    println(greet("Bob"))
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Kotlin directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).