Introduce vec<T> array type #6451
Description
This is a part of the Hack array proposal
A vec array is a container whose keys are zero-based continuous integers. This mirrors the behavior of the Vector class. This will be an array at runtime with extra restrictions to allow it to be used safely like a Vector 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 vec is constructed using the literal expression vec[].
$vec = vec[]; // Empty vec-array
$vec = vec['a', 'b', 'c']; // vec-array of 3 elementsThis is a static expression, so it can be used in a static initializer.
class C {
public static $vec = vec['a', 'b', 'c'];
}Traversable Constructor
A vec can also be constructed from any Traversable type, using the traversable constructor.
$array = array('a', 'b', 'c');
$vec = vec($array); // constructs vec-array from another arrayAdding Elements
To preserve the property of having continuous integer keys new elements can only be added to a vec using the array append operator.
$vec = vec[];
$vec[] = 1; // Allowed
$vec[0] = 2; // Allowed
$vec[10] = 3; // Error: 10 is out of bounds for the vec-arrayRemoving Elements
Using unset on a vec type will demote it to the dict type. If you want to preserve the vec property then you will need to use the various array functions that operate at the beginning or end of an array such as array_pop, array_push, array_shift, array_unshift, etc.
$vec = vec[1, 2, 3, 4];
array_pop($vec); // Now vec[1, 2, 3]
array_shift($vec); // Now vec[2, 3]
unset($vec[1]); // unsetting changes a vec to a dict[ 0 => 2]