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Functions

Function Declarations

Functions in Kotlin are declared using the fun keyword:

fun double(x: Int): Int {
    return 2 * x
}

Function Usage

Calling functions uses the traditional approach:

val result = double(2)

Calling member functions uses the dot notation:

Sample().foo() // create instance of class Sample and call foo

Parameters

Function parameters are defined using Pascal notation, i.e. name: type. Parameters are separated using commas. Each parameter must be explicitly typed:

fun powerOf(number: Int, exponent: Int) {
...
}

Default Arguments

Function parameters can have default values, which are used when a corresponding argument is omitted. This allows for a reduced number of overloads compared to other languages:

fun read(b: Array<Byte>, off: Int = 0, len: Int = b.size) {
...
}

Default values are defined using the = after type along with the value.

Overriding methods always use the same default parameter values as the base method. When overriding a method with default parameters values, the default parameter values must be omitted from the signature:

open class A {
    open fun foo(i: Int = 10) { ... }
}

class B : A() {
    override fun foo(i: Int) { ... }  // no default value allowed
}

If a default parameter precedes a parameter with no default value, the default value can be used only by calling the function with named arguments:

fun foo(bar: Int = 0, baz: Int) { /* ... */ }

foo(baz = 1) // The default value bar = 0 is used

But if a last argument lambda is passed to a function call outside the parentheses, passing no values for the default parameters is allowed:

fun foo(bar: Int = 0, baz: Int = 1, qux: () -> Unit) { /* ... */ }

foo(1) { println("hello") } // Uses the default value baz = 1 
foo { println("hello") }    // Uses both default values bar = 0 and baz = 1

Named Arguments

Function parameters can be named when calling functions. This is very convenient when a function has a high number of parameters or default ones.

Given the following function:

fun reformat(str: String,
             normalizeCase: Boolean = true,
             upperCaseFirstLetter: Boolean = true,
             divideByCamelHumps: Boolean = false,
             wordSeparator: Char = ' ') {
...
}

we could call this using default arguments:

reformat(str)

However, when calling it with non-default, the call would look something like:

reformat(str, true, true, false, '_')

With named arguments we can make the code much more readable:

reformat(str,
    normalizeCase = true,
    upperCaseFirstLetter = true,
    divideByCamelHumps = false,
    wordSeparator = '_'
)

and if we do not need all arguments:

reformat(str, wordSeparator = '_')

When a function is called with both positional and named arguments, all the positional arguments should be placed before the first named one. For example, the call f(1, y = 2) is allowed, but f(x = 1, 2) is not.

Variable number of arguments (vararg) can be passed in the named form by using the spread operator:

fun foo(vararg strings: String) { /* ... */ }

foo(strings = *arrayOf("a", "b", "c"))

Note that the named argument syntax cannot be used when calling Java functions, because Java bytecode does not always preserve names of function parameters.

Unit-returning functions

If a function does not return any useful value, its return type is Unit. Unit is a type with only one value - Unit. This value does not have to be returned explicitly:

fun printHello(name: String?): Unit {
    if (name != null)
        println("Hello ${name}")
    else
        println("Hi there!")
    // `return Unit` or `return` is optional
}

The Unit return type declaration is also optional. The above code is equivalent to:

fun printHello(name: String?) {
    ...
}

Single-Expression functions

When a function returns a single expression, the curly braces can be omitted and the body is specified after a = symbol:

fun double(x: Int): Int = x * 2

Explicitly declaring the return type is optional when this can be inferred by the compiler:

fun double(x: Int) = x * 2

Explicit return types

Functions with block body must always specify return types explicitly, unless it's intended for them to return Unit, in which case it is optional. Kotlin does not infer return types for functions with block bodies because such functions may have complex control flow in the body, and the return type will be non-obvious to the reader (and sometimes even for the compiler).

Variable number of arguments (Varargs)

A parameter of a function (normally the last one) may be marked with vararg modifier:

fun <T> asList(vararg ts: T): List<T> {
    val result = ArrayList<T>()
    for (t in ts) // ts is an Array
        result.add(t)
    return result
}

allowing a variable number of arguments to be passed to the function:

val list = asList(1, 2, 3)

Inside a function a vararg-parameter of type T is visible as an array of T, i.e. the ts variable in the example above has type Array<out T>.

Only one parameter may be marked as vararg. If a vararg parameter is not the last one in the list, values for the following parameters can be passed using the named argument syntax, or, if the parameter has a function type, by passing a lambda outside parentheses.

When we call a vararg-function, we can pass arguments one-by-one, e.g. asList(1, 2, 3), or, if we already have an array and want to pass its contents to the function, we use the spread operator (prefix the array with *):

val a = arrayOf(1, 2, 3)
val list = asList(-1, 0, *a, 4)

Infix notation

Functions can also be called using infix notations when

  • They are member functions or extension functions;
  • They have a single parameter;
  • They are marked with the infix keyword.
// Define extension to Int
infix fun Int.shl(x: Int): Int {
...
}

// call extension function using infix notation

1 shl 2

// is the same as

1.shl(2)

Function Scope

In Kotlin functions can be declared at top level in a file, meaning you do not need to create a class to hold a function, like languages such as Java, C# or Scala. In addition to top level functions, Kotlin functions can also be declared local, as member functions and extension functions.

Local Functions

Kotlin supports local functions, i.e. a function inside another function:

fun dfs(graph: Graph) {
    fun dfs(current: Vertex, visited: Set<Vertex>) {
        if (!visited.add(current)) return
        for (v in current.neighbors)
            dfs(v, visited)
    }

    dfs(graph.vertices[0], HashSet())
}

Local function can access local variables of outer functions (i.e. the closure), so in the case above, the visited can be a local variable:

fun dfs(graph: Graph) {
    val visited = HashSet<Vertex>()
    fun dfs(current: Vertex) {
        if (!visited.add(current)) return
        for (v in current.neighbors)
            dfs(v)
    }

    dfs(graph.vertices[0])
}

Member Functions

A member function is a function that is defined inside a class or object:

class Sample() {
    fun foo() { print("Foo") }
}

Member functions are called with dot notation:

Sample().foo() // creates instance of class Sample and calls foo

For more information on classes and overriding members see Classes and Inheritance.

Generic Functions

Functions can have generic parameters which are specified using angle brackets before the function name:

fun <T> singletonList(item: T): List<T> {
    // ...
}

For more information on generic functions see Generics.

Inline Functions

Inline functions are explained here.

Extension Functions

Extension functions are explained in their own section.

Higher-Order Functions and Lambdas

Higher-Order functions and Lambdas are explained in their own section.

Tail recursive functions

Kotlin supports a style of functional programming known as tail recursion. This allows some algorithms that would normally be written using loops to instead be written using a recursive function, but without the risk of stack overflow. When a function is marked with the tailrec modifier and meets the required form, the compiler optimises out the recursion, leaving behind a fast and efficient loop based version instead:

tailrec fun findFixPoint(x: Double = 1.0): Double
        = if (x == Math.cos(x)) x else findFixPoint(Math.cos(x))

This code calculates the fixpoint of cosine, which is a mathematical constant. It simply calls Math.cos repeatedly starting at 1.0 until the result doesn't change any more, yielding a result of 0.7390851332151607. The resulting code is equivalent to this more traditional style:

private fun findFixPoint(): Double {
    var x = 1.0
    while (true) {
        val y = Math.cos(x)
        if (x == y) return x
        x = y
    }
}

To be eligible for the tailrec modifier, a function must call itself as the last operation it performs. You cannot use tail recursion when there is more code after the recursive call, and you cannot use it within try/catch/finally blocks. Currently tail recursion is only supported in the JVM backend.