Functions: Introduction

The function keyword defines a global function.

function add_one(int $x): int {
  return $x + 1;

The function keyword can also be used to define methods.

Default Parameters

Hack supports default values for parameters.

function add_value(int $x, int $y = 1): int {
  return $x + $y;

This function can take one or two arguments. add_value(3) returns 4.

Required parameters must come before optional parameters, so the following code is invalid:

function add_value_bad(int $x = 1, int $y): int {
  return $x + $y;

Variadic Functions

You can use ... to define a function that takes a variable number of arguments.

function sum_ints(int $val, int ...$vals): int {
  $result = $val;
  foreach ($vals as $v) {
    $result += $v;
  return $result;

This function requires at least one argument, but has no maximum number of arguments.

// Passing positional arguments.
sum_ints(1, 2, 3);

// You can also pass a collection into a variadic parameter.
$args = vec[1, 2, 3];
sum_ints(0, ...$args);

Function Types

Functions are values in Hack, so they can be passed as arguments too.

function apply_func(int $v, (function(int): int) $f): int {
  return $f($v);

function usage_example(): void {
  $x = apply_func(0, $x ==> $x + 1);

Variadic functions can also be passed as arguments.

function takes_variadic_fun((function(int...): void) $f): void {
  $f(1, 2, 3);

  $args = vec[1, 2, 3];
  $f(0, ...$args);