[8.x] Add Arr::isList() method

[PHLAK]

Added the Arr::isList() method for determining if an array is a "list". An array is a "list" if all array keys are sequential integers starting from 0 with no gaps in between.

This is a polyfilled version of the array_is_list() function that is coming to PHP with v8.1 (currently unreleased). Additionally, Laravel 8 supports PHP versions back to 7.3 so this PR brings this functionality to applications still on these versions.

Implementation

This implementation uses the polyfill code as described in the RFC proposal which avoids potential issues when using other methods (e.g. array_values($array) === $array) for determining "listness".

Future Considerations

In the future this method can be refactored to use the native array_is_list() function when support for PHP versions < 8.1 are dropped. Alternatively the method may be removed all together.

[taylorotwell]

Can you give an example where ! Arr::isAssoc(...) wouldn't work correctly for this use case?

[JacekAndrzejewski]

@taylorotwell The RFC describes cases where obvious implementation (which is used in isAssoc) won't work. Those would be rare, but technically could happen and would be hard to debug.

// Pitfalls of simpler polyfills - NAN !== NAN
$x = ['key' => 2, NAN];
unset($x['key']);
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

[taylorotwell]

Arr::isAssoc does not use array_values so just want to confirm.

[taylorotwell]

[JacekAndrzejewski]

That's what happens when you read code tired -_-
You're right. And I can't come up with any case where it wouldn't work, as keys are cast and no NAN can be a key. Calling array_keys twice will always create an array of numbers 0 to n, and one call will always create array of strings or ints. So comparison will work as expected every time.

One thing to consider would be performance for huge arrays, and without tests I can't even give a guess which would be faster. Probably if array is million of elements long and last key is wrong then array_keys would be faster.

[taylorotwell]

@PHLAK would it be better to just convert this method to return ! Arr::hasList($array) until we can use array_is_list?

[PHLAK]

@taylorotwell I assume you meant return ! Arr::isAssoc($array). This would be fine as long as we can't identify a case that breaks this (I can't). I went ahead and made this change.