Spread Operator in Array Expression

• Version: 0.2
• Date: 2018-10-13
• Author: CHU Zhaowei, jhdxr@php.net
• Status: Under discussion
• First Published at: http://wiki.php.net/rfc/spread_operator_for_array

PHP has already supported argument unpacking (AKA spread operator) since 5.6. This RFC proposes to bring this feature to array expression.

An array pair prefixed by ... will be expanded in places during array definition. Only arrays and objects who implement Traversable can be expanded.

For example,

$parts = ['apple', 'pear'];
$fruits = ['banana', 'orange', ...$parts, 'watermelon'];
// ['banana', 'orange', 'apple', 'pear', 'watermelon'];

It's possible to do the expansion multiple times, and unlike argument unpacking, ... can be used anywhere. It's possible to add normal elements before or after the spread operator.

Spread operator works for both array syntax(array()) and short syntax([]).

$arr1 = [1, 2, 3];
$arr2 = [...$arr1]; //[1, 2, 3]
$arr3 = [0, ...$arr1]; //[0, 1, 2, 3]
$arr4 = array(...$arr1, ...$arr2, 111); //[1, 2, 3, 1, 2, 3, 111]

In order to make the behavior consistent with argument unpacking, string keys are not supported. A recoverable error will be thrown once a string key is encountered.

It's not possible to unpack an array by reference.

$arr1 = [1, 2, 3];
$arr2 = [...&$arr1]; //invalid syntax

However, if elements in the array to be unpacked are stored by reference, they will be stored by reference in the new array as well.

$one = 1;
$arr1 = [&$one, 2, 3];
$arr2 = [0, ...$arr1];
var_dump($arr2);
/*
array(4) {
  [0]=>
  int(0)
  [1]=>
  &int(1)
  [2]=>
  int(2)
  [3]=>
  int(3)
}
*/

This change should not break anything.

1. Spread operator should have a better performance than array_merge. It's becuase not only that spread operator is a language structure while array_merge is a function call, but also compile time optimiztion can be performaned for constant arrays.
2. array_merge only supports array, while spread operator also supportes objects implementing Traversable.

This is kind of out of scope here to discuss other concat / merge operation. The important thing is we should make the behavior of same operator consistent and not to confuse userland developer. It's also why I changed the behavior for string keys in this revised version.

next PHP 7.x, likely 7.4

I'm not sure if this RFC will have any impact on Opcache. I'll check it after I finish the patch.

As this is a language change, a 2/3 majority is required.

WIP

https://github.com/php/php-src/pull/3640

After the project is implemented, this section should contain

1. the version(s) it was merged into
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature
4. a link to the language specification section (if any)

Links to external references, discussions or RFCs

Keep this updated with features that were discussed on the mail lists.