This is a part of the Hack array proposal

A keyset is a set that can only contain valid array keys. It is implemented as a dict whose values and keys are the same. For example:

$set = keyset[1, 2, 3];
// Logically expands to
dict[
  1 => 1,
  2 => 2,
  3 => 3,
]

This will be an array at runtime with extra restrictions to allow it to be used safely like a Set from the type checkers perspective. This focuses mainly on the desired runtime behavior, but if there are questions for how it will interact with the type checker I'll be happy to discuss.

Literal Constructor

A keyset is constructed using the literal expression keyset[].

$set = keyset[]; // Empty set-array
$set = keyset['a', 'b', 'c']; // set-array of 3 elements

This is a static expression, so it can be used in a static initializer.

class C {
  public static $set = keyset['a', 'b', 'c'];
}

Traversable Constructor

A keyset can also be constructed from any Traversable type, using the traversable constructor.
We will also have a function keyset_from_keys to construct a keyset from the keys of a KeyedTraversable.

$array = array('a', 'b', 'c');
$set = keyset($array); // constructs set-array from another array

Checking Membership

isset is used to check membership in a keyset

$set = keyset[4, 5, 6];
isset($set[0]);

Adding Elements

To add a new element to a keyset we will introduce a new “set append” operator. From the type checkers perspective setting a key directly will be an error, but the runtime will behave the same as a dict.

$set = keyset[];
$set{} = 2; // "Set-append" that sets (2 => 2) in $set
$set[$x] = 4; // Type error, but not a runtime error

Removing Elements

Removing an element from a keyset is identical to removing an element from a PHP array by using the unset function.