Kotlin Design Patterns and Best Practices/

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Closures and Pure Functions

Closures and Pure Functions

Learn about closures and pure functions in Kotlin.

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In the object-oriented paradigm, state is always stored within objects. But in functional programming, this isn’t necessarily the case. Let’s look at the following function as an example:

fun counter(): () -> Int {
    var i = 0
    return { i++ }
}
A function returns a lambda that counts up each time it's called

The preceding example is clearly a higher-order function, as we can see by its return type. It returns a function with zero arguments that produces an integer.

Let’s store it in a variable, and invoke it multiple times:

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fun main() {
    fun counter(): () -> Int {
        var i = 0
        return { i++ }
    }
    val next = counter()
    println(next()) 
    println(next()) 
    println(next()) 
}

As we can see, the function is able to keep a state, in this case, the value of a counter, even though it is not part of an object.

This is called a closure. The lambda has access to all of the local variables of the function that wraps it, and those local variables persist, as long as the ...