Introduction

Hack includes diverse range of array-like data structures.

Hack arrays are value types for storing iterable data. The types available are vec, dict and keyset. When in doubt, use Hack arrays.

Hack collections are deprecated object types for storing iterable data. The types available include Vector, Map, Set, Pair and helper interfaces.

Quick Start

You can create Hack arrays as follows:

$v = vec[2, 1, 2];

$k = keyset[2, 1];

$d = dict['a' => 1, 'b' => 3];

The Hack Standard Library

There are many helpful functions in the C, Vec, Keyset and Dict namespaces, which are a part of the Hack Standard Library (HSL).

For more information on included HSL namespaces, see Hack Standard Library: Namespaces.

// The C namespace contains generic functions that are relevant to
// all array and collection types.
C\count(vec[]); // 0
C\is_empty(keyset[]); // true

// The Vec, Keyset and Dict namespaces group functions according
// to their return type.
Vec\keys(dict['x' => 1]); // vec['x']
Keyset\keys(dict['x' => 1]); // keyset['x']

Vec\map(keyset[1, 2], $x ==> $x + 1); // vec[2, 3]

Arrays Cheat Sheet

Operation	vec	dict	keyset
Initialize empty	$v = vec[];	$d = dict[];	$k = keyset[];
Literal	$v = vec[1, 2, 3];	$d = dict['foo' => 1];	$k = keyset['foo', 'bar'];
From Another Container*	$v = vec($container);	$d = dict($keyed_container);	$k = keyset($container);
Keys from Container*	$v = Vec\keys($container);	N/A	$k = Keyset\keys($container);
Add Elements	$v[] = 4;	$d['baz'] = 2;	$k[] = 'baz';
Bulk Add Elements	$v = Vec\concat($t1, $t2)	$d = Dict\merge($kt1, $kt2)	$k = Keyset\union($t1, $t2)
Remove Elements	Remove-at-index is unsupported; Vec\drop($v,$n), Vec\take($v,$n); $first=C\pop_front(inout $x), $last=C\pop_back(inout $x)	unset($d['baz']);	unset($k['baz']);
Key Existence	C\contains_key($v, 1)	C\contains_key($d, 'foo')	C\contains_key($k, 'foo')
Value Existence	C\contains($v, 3)	C\contains($d, 2)	Use C\contains_key($k, 'foo')
Equality (Order-Dependent)	$v1 === $v2	$d1 === $d2	$k1 === $k2
Equality (Order-Independent)	N/A	Dict\equal($d1, $d2)	Keyset\equal($k1, $k2)
Count Elements (i.e., length, size of array)	C\count($v)	C\count($d)	C\count($k)
Type Signature	vec<Tv>	dict<Tk, Tv>	keyset<Tk>
Type Refinement	$v is vec<_>	$d is dict<_, _>	$k is keyset<_>
Awaitable Consolidation	Vec\from_async($v)	Dict\from_async($d)	Keyset\from_async($x)

* $container can be a Hack Array or Hack Collection

Arrays Conversion Cheat sheet

Prefer to use Hack arrays whenever possible. When interfacing with legacy APIs that expect older Containers, it may be easier to convert. Here's how:

Converting	To Vector	To Map	To Set
dict	N/A	new Map($d)	N/A
dict keys	Vector::fromKeysOf($d)	N/A	Set::fromKeysOf($d)
dict values	new Vector($d)	N/A	new Set($d)
vec	new Vector($v)	new Map($v)	new Set($v)
keyset	new Vector($k)	new Map($k)	new Set($k)