[Mutator]: Spread operator in Array Expression - leave only the first element

[maks-rafalko]

RFC: https://wiki.php.net/rfc/spread_operator_for_array
PHP 7.4+

• Covered by tests
• Doc PR: Mutator: Spread operator in Array Expression - docs site#134

This mutator does the following mutations:

- $a = [...[1, 2, 3], 4];
+ $a = [[1, 2, 3][0], 4];

- $a = [...$collection, 4];
+ $a = [$collection[0], 4];

- $a = [...getCollection(), 4];
+ $a = [getCollection()[0], 4];

The only one issue I think will be with this mutator, is that it can work with any \Traversable, including \Generator, and the following mutation will be self-killed:

function arrGen() {
    for($i = 11; $i < 15; $i++) {
        yield $i;
    }
}

- $arr = [...arrGen()];
+ $arr = [arrGen()[0]];

Produces an error:

Fatal error: Uncaught Error: Cannot use object of type Generator as array in ...

What can we do?

1. The first and the best solution is to exactly know what is the return type hint of arrGen() function and knowing that, do either arrGen()[0] for array or iterator_to_array(arrGen())[0] for iterator. However, unfortunately, Infection is not able to guess types at the moment (as for example PHPStan/Psalm do it)

2. The second option is to always do such mutation

- $arr = [...arrGen()];
+ $_traversable = arrGen();
+ $arr = [is_array($_traversable) ? $_traversable[0] : iterator_to_array($_traversable))[0];

3. Do not mutate variables and functions and methods, only raw arrays (the very first diff above)

Thoughts?

[sanmai]

Great idea! I'm all in for the variant with an is_array check. It shall be the most useful.

[maks-rafalko]

@sanmai no problem, implemented. The new mutations are:

- $a = [...[1, 2, 3], 4];
+ $a = [[1, 2, 3][0], 4];

- $a = [...$collection, 4];
+ $a = [is_array($collection) ? $collection[0] : iterator_to_array($collection)[0], 4];

- $a = [...getCollection(), 4];
+ $a = [is_array(getCollection()) ? getCollection()[0] : iterator_to_array(getCollection())[0], 4];

[sanmai]

I wish iterator_to_array would take arrays, returning arrays as they were! Oh well...

It may be better to use reset() because PHP will happily unroll non-zero indexed arrays.

$array = [1 => 'bar','baz'];
$example = [...$array, 'foo'];

[maks-rafalko]

makes sense, updated the code

[maks-rafalko]

Unfortunately, reset(arrGen()) produces a notice:

Notice: Only variables should be passed by reference

I see only one solution - introduce a variable:

- $arr = [...arrGen()];
+ $_traversable = arrGen();
+ $arr = [is_array($_traversable) ? reset($_traversable) : reset(iterator_to_array($_traversable)));

UPD:

we can also use array_slice that makes new keys: https://3v4l.org/JKM6J

function getArray() {
    return [1 => 11, 2 => 22];
}

array_slice(getArray(), 0, 1)[0]; // 11

and the diff

- $arr = [...arrGen()];
+ $arr = [array_slice(is_array($_traversable) ? $_traversable : iterator_to_array($_traversable), 0, 1)[0]];

[maks-rafalko]

Updated to use array_slice as discussed above. For integer keys, array_slice returns a new array with reset indexes, started from 0 and we safely using array_slice(...)[0].

If the array has string keys, it's not a problem, because spread operator can not work with string keys.

[sanmai]

Yes, otherwise put you can't use a spread operator with such an array in the first place.

Do you think this can work?

$a = [[...$collection][0], 4];

[maks-rafalko]

Do you think this can work?

$a = [[...$collection][0], 4];

Not sure I got the question. If this is the original code before mutation, then the current state of PR works nice with it:

$collection = [5, 6, 7];

- $a = [[...$collection][0], 4];
+ $a = [[array_slice(is_array($collection) ? $collection : iterator_to_array($collection), 0, 1][0], 4];

which results to

[[5][0], 4]

// and then 

[5, 4]

[sanmai]

I'm more about this mutation:

- $a = [...$collection, 4];
+ $a = [[...$collection][0], 4];

- $a = [...getCollection(), 4];
+ $a = [[...getCollection()][0], 4];

If it works, we should be able to cut on array_slice() and so forth.

[maks-rafalko]

👏 awesome

Updated the code and added more tests for [...new ArrayIterator(['a', 'b', 'c'])] case from RFC

[maks-rafalko]

Appveyor fail is unrelated, see #786

[maks-rafalko]

Thank you @sanmai for review and ideas