PHP RFC: Add array_is_list(array $array): bool


PHP's array data type is rare in that it supports both integer and string keys, and that iteration order is guaranteed. While it is possible to efficiently check that something is an array, that array may be an associative array, have missing array offsets, or contain out of order keys. It can be useful to verify that the assumption that array keys are consecutive integers is correct, both for data that is being passed into a module or for data being returned by a module. In serializers, it may also be useful to have an efficient check to distinguish lists from associative arrays - for example, json_encode does this when deciding to serialize a value as [0, 1, 2] instead of {“0”:0,“2”:1,“1”:1} for arrays with different key orders.


Add a new function array_is_list(array $array): bool that will return true if the array keys are 0 .. count($array)-1 in that order. For other arrays, it returns false. For non-arrays, it throws a TypeError.

This RFC doesn't change PHP's type system and doesn't add new type hints.

The functionality is equivalent to the below polyfill:

function array_is_list(array $array): bool {
    $expectedKey = 0;
    foreach ($array as $i => $_) {
        if ($i !== $expectedKey) { return false; }
    return true;
$x = [1 => 'a', 0 => 'b'];
var_export(array_is_list($x));  // false because keys are out of order
var_export(array_is_list($x));  // true
// Pitfalls of simpler polyfills - NAN !== NAN
$x = ['key' => 2, NAN];
var_export($x === array_values($x));  // false because NAN !== NAN
var_export($x);  // array (0 => NAN)
var_export(array_is_list($x));  // true because keys are consecutive integers starting from 0
array_is_list(new stdClass());  // throws a TypeError
array_is_list(null);  // throws a TypeError

Note that there are pitfalls in writing a correct polyfill/substitute. For example, array_values($array) === $array would be false for some arrays containing NAN, and array_keys($array) === range(0, count($array) - 1) is wrong for the empty array.

The native implementation will quickly return true for most lists by checking the C macro HT_IS_PACKED(array) && HT_IS_WITHOUT_HOLES(array). This optimization is already used by json_encode().

Example Use Cases

  1. Having an efficient, correct, and readable way to check that an array is actually a list (that doesn't have the pitfalls mentioned earlier).
  2. Making it more efficient and straightforward to check assumptions about data (In most other languages, there is already a non-associative list/array type that could be enforced during compilation or at runtime with the equivalent of instanceof)
  3. Throwing or warning in a library, framework, or API if the passed in value is not a list with elements in order and without gaps. For example, a potential source of bugs is that array_filter($list) returns a list with gaps in it, and array_values(array_filter($list)) should be used instead.
  4. Serializers or data encoders written in PHP, or other use cases that require or benefit from checking if data conforms to an expected format.
  5. Detecting invoking a function with named arguments in variable arguments in code that does not expect named arguments (e.g. example(argname: $value); for function example(...$args) {}).

Proposed PHP Version


RFC Impact

To Opcache

Opcache's architecture does not change because the type system is unchanged; optimizations of array_is_list() can easily be added or removed.

In the RFC's implementation, opcache evaluates the call array_is_list(arg) to a constant if the argument is a constant value and doesn't throw (same mechanism currently used for array_keys, etc.).

Long-term, if this sees wide enough adoption to affect performance on widely used apps or frameworks, opcache's contributors will have the option of adding additional checks to make opcache infer that array_is_list() being true implies that the keys of the array are integers.

(Currently, Opcache only optimizes type checks that are converted to type check opcodes such as is_resource() and is_array(). Opcache doesn't do anything similar for opcodes that become regular function calls such as is_numeric(), so the implementation for array_is_list() included with this RFC does not do this.)


Possibility of naming conflicts with future vector-like types

Originally, this was called is_list, but renamed due to the potential of naming conflicts with a potential list type.


If we do eventually end up with list/vec types, would the naming here conflict at all? Or would it cause confusion and name collision? (Insert name bikeshedding here.)

There's definitely the potential for naming conflicts if the type is called list but not if it's called vec/vector/varray similar to https://docs.hhvm.com/hack/built-in-types/arrays - I'd strongly prefer the latter if there was a viable implementation and it used sequential memory instead of a linked list.

If the type is named list instead of vec and ends up incompatible with arrays, there'd need to be an is_list_type($val) or $val is list or some other new type check with a less preferable name. If it's compatible with arrays/lists (e.g. only checked during property assignment, passing in arguments, and returning values), then it wouldn't be an issue.

- array_is_list(array $array) is consistent with many other array_* methods, which only accept arrays. - It is very possible that we may end up using the word list anyway despite those objections, because it's already a reserved keyword in PHP for unrelated syntax (list($first, $second) = $values). Recently added types such as object, void, and iterable (and scalar types) were added in previous PHP versions despite not being reserved in the past. - The name vector may conflict with the php-ds PECL depending on how functionality is implemented.

Providing objects with APIs similar to the external PECL https://www.php.net/manual/en/class.ds-vector.php and the SPL may be easier to adopt because it can be polyfilled, but there's the drawback that there aren't the memory savings from copy-on-write and that there's the performance overhead of method calls to offsetGet(), etc.

As mentioned in Changes to PHP's type system, I'd expect the addition of a separate/incompatible vector type to be a massive undertaking, and possibly unpopular if it splits the language. In Hack/HHVM, it was practical for users to adopt because HHVM is bundled with a typechecker that checks that the uses are correct at compile time - because PHP has no bundled type checker, a new type would potentially cause a lot of unintuitive behaviors.

Additionally, a name of is_list may cause confusion with built-in list types such as SplDoublyLinkedList.


Voting started on 2021-01-06 and ended 2021-01-20

This is a Yes/No vote, requiring a 2/3 majority

Add the function array_is_list(array $array): bool to PHP?
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Final result: 41 1
This poll has been closed.


Rejected Features

Alternate names

is_sequential_array/array_is_sequential was rejected because [2=>'a', 3=>'b'] is also sequential.

is_zero_indexed_array/array_is_zero_indexed was rejected because that term is much less commonly used.

Alternate implementations

The signature is_array_and_list(mixed $value): bool was considered, but rejected because silently returning false for objects would be surprising, and the behavior for future list-like types might be misunderstood (SplDoublyLinkedList, ArrayObject, etc.)

This deliberately only returns true for arrays with sequential keys and a start offset of 0. It returns false for [1=>'first', 2=>'second'].

This deliberately throws a TypeError for non-arrays.

Adding flags to is_array()


I actually like the idea of flags added to is_array() for this.

Something like:


I’m not suggesting these names; they’re for illustration only.

I'm strongly opposed to adding any flags to is_array - keeping basic type checks simple would help in learning/reading/remembering the language. The addition of flags has a small impact on performance for calls that aren't unambiguously qualified (especially if using both), and it makes it harder to see issues like is_array(really_long_multiline_call(arg1, arg2, ZERO_INDEXED)) where ZERO_INDEXED is passed to another function instead of is_array.

Changes to PHP's type system

This RFC does not attempt to change php's type system. External static analyzers may still benefit from inferring key types from array_is_list() conditionals seen in code - array_is_list() conditionals would give more accurate information about array keys that can be used to detect issues or avoid false positives. (Phan, Psalm, and PHPStan are all static analyzers that support the unofficial phpdoc type list<T>, which is used for arrays that would satisfy array_is_list()).

Any attempt to change php's type system would need to deal with references and the global scope - e.g. what would happen if an array was passed to list &$val but modified to become a non-list from a different callback or through asort().

Additionally, I'd personally expect that changes to the type system that were backwards incompatible would be possible, but unpopular and difficult to implement. HHVM is a project that was initially compatible with php, but has recently dropped compatibility with PHP. https://docs.hhvm.com/hack/built-in-types/arrays may be of interest to anyone who is interested in ways to migrate to stricter alternatives to php's arrays, but that required an entirely different language mode to use (<?hh), which doesn't seem viable for PHP itself (for reasons such as splitting the ecosystem and being incompatible with older php versions).

The thread https://externals.io/message/109760#109812 discussed this, but I'm not aware of anyone working on an implementation of list/vec, and supporting adding list/vec to the type system would be a lot of work for PECL extensions, language design, backwards compatibility concerns, etc. (It would also potentially be an issue with serializing/unserializing for data sent to/from older php versions (e.g. memcache, $_SESSION data, etc.))

Hack introduced the vec type (with value semantics) in 2016 after they'd experimented first with Vector (object semantics). Use of Vector is now discouraged.

Details here: https://github.com/facebook/hhvm/issues/6451

FB/Hack appears to be in the multi-year process of moving all PHP arrays to one of [vec/dict/keyset]. That's likely not an option for PHP itself, but having the option of a vec equivalent (in this proposal “list”) would make sense, I think.


Most users don't realize that PHP's arrays-not-really-arrays have caused millions of dollars in security breaches in the past. :-) They're dangerous and to be avoided whenever possible.

I'm very open to a list/sequence type, but as others have noted there's a whole crapload of details to sort out to make it viable. In particular:

  • Is it an effective subclass of array? IMO, no. It should have absolutely no auto-conversion to/from an array whatsoever of any kind, period. Keep them as separate as possible.
  • Should it even have random-access indexes? Honestly I'd say no; Just support adding, removing, and iteration and generate the indexes on the fly when iterating if necessary.
  • Should they pass like arrays or like objects? Many questions here.
  • Should they be mutable or immutable? I could argue for either one effectively, I think, though I'd honestly favor immutable.
  • Are they iterable? Presumably, but does that have any weird implications for iterables that implicitly assume there are keys? How's that work?
  • Does it make sense to add them without type enforcement via generics? Lists + Generics would be lovely, but as we've seen Generics are Hard(tm) and Not Imminent(tm). But would adding them now make a generic version harder in the future? (I've no idea.)
  • Besides add/remove/iterate, what other baked-in functionality should they have? Eg, can they be mapped/filtered/reduced? It would really suck to revisit lists and not fix that disconnect in the API. (Insert me talking about comprehensions and stuff here.) Ideally this would happen as part of a larger review of how collections work at various levels, which are currently highly clunky.

Those are all solvable problems (and I've likely forgotten several), but they would have to be thought through extensively before an implementation could be viable.


  • 0.3: Change name and signature from is_array_and_list(mixed $value) to array_is_list(array $array)
  • 0.2: Rename from is_list() to is_array_and_list(), add references and more rejected features
rfc/is_list.txt · Last modified: 2021/01/20 23:51 by tandre