Hack
Hack lets you write code quickly, while also having safety features built in, like static typechecking.
To run Hack code, install HHVM, the open-source virtual machine.
/* ==================================
* READ THE DOCS!
* ==================================
*/
/* For more information on the Hack language:
* - About Hack: https://hacklang.org/
* - Documentation: https://docs.hhvm.com/hack/
*/
/* ==================================
* A NOTE ON PHP
* ==================================
*/
// The Hack language began as a superset of PHP.
// Since then, the languages have (largely) diverged.
// You may encounter the .php extension, which is no longer recommended.
/* ==================================
* COMMENTS
* ==================================
*/
// Hack has single-line comments...
/* Multi-line comments...
*
*/
/**
* ... and a special syntax for doc comments.
*
* Use doc comments to summarize the purpose of a definition, function, class or method.
*/
/* ==================================
* NAMESPACES
* ==================================
*/
// Namespaces contain definitions of classes, interfaces, traits, functions, and constants.
namespace LearnHackinYMinutes {
/* ==================================
* TYPES
* ==================================
*/
function demo_hack_types(): void {
// Hack has five primitive types: bool, int, float, string, and null.
$is_helpful = true; // bool
$int_value = 10; // int
$precise_value = 2.0; // float
$hello_world = "Hello World!"; // string
$null_string = null; // null
// Create a `shape` with the shape keyword, with a series of field names and values.
$my_point = shape('x' => -3, 'y' => 6, 'visible' => true);
// Create a `tuple` with the tuple keyword, with a series of two or more types as values.
$apple_basket = tuple("apples", 25); // different types are OK
// Use `arraykey` to represent either an integer or string.
$the_answer = 42;
$is_answer = process_key($the_answer);
// Similarly, `num` represents either an int or float.
$lucky_number = 7;
$lucky_square = calculate_square($lucky_number);
}
function process_key(arraykey $the_answer): bool {
if ($the_answer is int) {
return true;
} else {
return false;
} // true
}
function calculate_square(num $arg)[]: float {
return ((float)$arg * $arg);
}
// Enums are limited to int or string (as an Arraykey), or other enum values.
enum Permission: string {
Read = 'R';
Write = 'W';
Execute = 'E';
Delete = 'D';
}
// In contrast, an enum class can be of any value type!
enum class Random: mixed {
int X = 42;
string S = 'foo';
}
/* ==================================
* HACK ARRAYS
* ==================================
*/
// The following line lets us use functions in the `C\` namespace.
use namespace HH\Lib\C; // the `C` library operates on containers
function demo_hack_arrays(): void {
// vec: ordered
$v = vec[1, 2, 3];
$letters = vec['a', 'b', 'c'];
$letters[0]; // returns 'a'
$letters[] = 'd'; // appends 'd'
// `inout` provides pass-by-reference behavior
C\pop_back(inout $letters); // removes 'd'
C\pop_front(inout $letters); // removes 'a'
// keyset: ordered, without duplicates
$k = keyset[1, 2, 3]; // values must be int or string
$colors = keyset['red', 'blue', 'green'];
// keyset keys are identical to their values
$colors['blue']; // returns 'blue'.
$colors[] = 'yellow'; // appends 'yellow'
unset($colors['red']); // removes 'red'
// dict: ordered, by key-value
$d = dict['a' => 1, 'b' => 3]; // keys must be int or string
$alphabet = dict['a' => 1, 'b' => 2];
$alphabet['a']; // indexing at 'a' returns `1`
$alphabet['c'] = 3; // adds a new key-value pair of `c => 3`
unset($alphabet['b']); // removes 'b'
}
/* ==================================
* THE HACK STANDARD LIBRARY (HSL)
* ==================================
*/
// The Hack Standard Library is a set of functions and classes for the Hack language.
// Namespace use declarations are ideally at the top of your file but are placed here for instruction purposes.
use namespace HH\Lib\Str; // The `Str` library operates on strings
function demo_hack_standard_library(): void {
$letters = vec['a', 'b', 'c'];
$colors = keyset['red', 'blue', 'green'];
$alphabet = dict['a' => 1, 'b' => 2];
C\contains($letters, 'c'); // checks for a value; returns 'true'
C\contains($colors, 'purple'); // checks for a value; returns 'false'
C\contains_key($alphabet, 'a'); // checks for a key; returns 'true'
C\contains($alphabet, 'd'); // checks for a value; returns 'false'
Str\length("foo"); // returns `3`
Str\join(vec['foo', 'bar', 'baz'], '!'); // returns `foo!bar!baz`
}
/* ==================================
* HELLO WORLD!
* ==================================
*/
use namespace HH\Lib\IO; // the `IO` library is a standard API for input / output
<<__EntryPoint>> // required attribute for the typical entry/main function
async function main(): Awaitable<
void,
> { // does not need to be named 'main' / is an asynchronous function
await IO\request_output()->writeAllAsync(
"Hello World!\n",
); // prints 'Hello World'!
}
/* ==================================
* FUNCTIONS
* ==================================
*/
// Functions are defined globally.
// When a function is defined in a class, we refer to the function as a method.
// Functions have return types (here: `int`) and must return a value of
// that type or return no value when a void return type annotation was used.
function add_one(int $x): int {
return $x + 1;
}
// Functions can also have defined, default values.
function add_value(int $x, int $y = 1): int {
return $x + $y;
}
// Functions can be variadic (unspecified length of arguments).
function sum_ints(int $val, int ...$vals): int {
$result = $val;
foreach ($vals as $v) {
$result += $v;
}
return $result;
}
// Functions can also be anonymous (defined with the `==>` arrow).
// $f = (int $x): int ==> $x + 1;
/* ==================================
* PIPE OPERATOR
* ==================================
*/
// The pipe operator, `|>`, evaluates the result of a left-hand expression
// and stores the result in `$$`, the predefined pipe variable.
use namespace HH\Lib\Vec;
function demo_pipe_operator(): void {
Vec\sort(Vec\map(vec[2, 1, 3], $a ==> $a * $a)); // vec[1,4,9]
// the same result, but using the pipe operator and pipe variable:
$x = vec[2, 1, 3]
|> Vec\map($$, $a ==> $a * $a) // $$ with value vec[2,1,3]
|> Vec\sort($$); // $$ with value vec[4,1,9]
}
/* ==================================
* ATTRIBUTES
* ==================================
*/
// Hack provides built-in attributes that can change runtime or static type checking behavior.
// For example, we used the `__EntryPoint` attribute earlier in the "Hello World!" example.
// As another example, `__Memoize` caches the result of a function.
<<__Memoize>>
async function do_expensive_task(): Awaitable<string> {
$site_contents = await \HH\Asio\curl_exec("http://hacklang.org");
return $site_contents;
}
/* ==================================
* CONTEXTS
* ==================================
*/
// Hack functions are attached to different contexts and capabilities.
// A context is a grouping of capabilities; that is, a grouping of permissions.
// To declare allowed contexts (and capabilities), use the Context List `[]`.
// If contexts are not defined, your function includes permissions defined in Hack's `defaults` context.
// Because the context list is NOT defined, the `defaults` context is implicitly declared.
async function implicit_defaults_context(): Awaitable<void> {
await IO\request_output()->writeAllAsync(
"Hello World!\n",
); // prints 'Hello World'!
}
// In the function below, the context list is defined to have the `defaults` context.
// A function can have multiple contexts [context1, context2, ...].
// `defaults` includes most of the capabilities defined by the Hack language.
async function explicit_defaults_context()[defaults]: Awaitable<void> {
await IO\request_output()->writeAllAsync("Hello World!\n");
}
// You can also specify zero contexts to create a pure function (no capabilities).
async function empty_context()[]: Awaitable<void> {
// The following line is an error, as the function does not have IO capabilities.
// await IO\request_output()->writeAllAsync("Hello World!\n");
}
/* ==================================
* GENERICS
* ==================================
*/
// Generics allow classes or methods to be parameterized to any set of types.
// That's pretty cool!
// Hack typically passes by value: use `inout` to pass by reference.
function swap<T>(inout T $input1, inout T $input2): void {
$temp = $input1;
$input1 = $input2;
$input2 = $temp;
}
/* ==================================
* CLASSES
* ==================================
*/
// Classes provide a way to group functionality and state together.
// To define a class, use the `class` keyword. To instantiate, use `new`.
// Like other languages, you can use `$this` to refer to the current instance.
class Counter {
private int $i = 0;
public function increment(): void {
$this->i += 1;
}
public function get(): int {
return $this->i;
}
}
// Properties and Methods can be static (not requiring instantiation).
class Person {
public static function favoriteProgrammingLanguage(): string {
return "Hack";
}
}
function demo_hack_classes(): void {
// Use `new` to instantiate a class.
$c1 = new Counter();
// To call a static property or method, use `::`
$typical_person = tuple("Andrew", Person::favoriteProgrammingLanguage());
}
// Abstract class can be defined, but not instantiated directly.
abstract class Machine {
public function openDoors(): void {
return;
}
public function closeDoors(): void {
return;
}
}
/* ==================================
* INTERFACES
* ==================================
*/
// A class can implement a set of requirements via an interface.
// An interface is a set of method declarations and constants.
interface Plane {
// A constant is a named value. Once defined, the value cannot be changed.
const MAX_SPEED = 300;
public function fly(): void;
}
/* ==================================
* TRAITS
* ==================================
*/
// A trait defines properties and method declarations.
// Traits are recommended when abstracting code for reuse.
// Traits are included in code via the `use` keyword.
trait Airplane {
// Introduce a class or interface requirement with the following syntax:
require extends Machine; // abstract class
require implements Plane; // interface
public function takeOff(): void {
$this->openDoors();
$this->closeDoors();
$this->fly();
}
}
class Spaceship extends Machine implements Plane {
use Airplane;
public function fly(): void {
// fly like the wind
}
}
/* ==================================
* KEEP READING!
* ==================================
*/
/* This is a simplified guide!
* There's much more to learn, including:
* - Asynchronous Operations: https://docs.hhvm.com/hack/asynchronous-operations/introduction
* - Reified Generics: https://docs.hhvm.com/hack/reified-generics/reified-generics
* - XHP: https://docs.hhvm.com/hack/XHP/setup
* - ... and more!
*/
}More Information #
Visit the Hack language reference to learn more about the Hack language.
For more information on HHVM, including installation instructions, visit the official HHVM website.
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