Transducers

transducers.php

<?php

declare(strict_types=1);

namespace Transducers;

use Closure;

function compose(array $reducers): Closure
{
    $reducers = array_reverse($reducers);

    return fn (Closure $step) =>
        fn ($accumulator, $current) =>
            array_reduce(
                $reducers,
                fn ($accumulator, $current) => $current($accumulator),
                $step
            )($accumulator, $current);
}

function filter(Closure $predicate): Closure
{
    return fn (Closure $step) =>
        fn ($accumulator, $current) =>
            $predicate($current)
                ? $step($accumulator, $current)
                : $accumulator;
}

function map(Closure $callback): Closure
{
    return fn (Closure $step) =>
        fn ($accumulator, $current) =>
            $step($accumulator, $callback($current));
}

usage.php

<?php

declare(strict_types=1);

use function Transducers\compose;
use function Transducers\filter;
use function Transducers\map;

$double = fn ($value) => $value * 2;
$isEven = fn ($value) => ($value % 2) === 0;

$doubleEvens = compose([
    filter($isEven),
    map($double),
]);

$xform = $doubleEvens(function ($accumulator, $current) {
    $accumulator[] = $current;

    return $accumulator;
});

$result = array_reduce([2, 2, 3, 4], $xform, []);

// [4, 4, 8]

This was fun. Do not recommend.