it.php

<?php

declare(strict_types=1);

/**
 * @template T
 * @param T $x
 * @return T
 */
function id(mixed $x): mixed
{
    return $x;
}

final readonly class It implements ArrayAccess
{
    private Closure $function;

    public function __construct(?Closure $function = null)
    {
        $this->function = $function ?? id(...);
    }

    public function __invoke(mixed $result): mixed
    {
        return ($this->function)($result);
    }

    public function __get(string $name)
    {
        return new self(
            fn(mixed $result): mixed => ($this->function)($result)->{$name},
        );
    }

    public function __call(string $name, array $arguments)
    {
        return new self(
            fn(mixed $result): mixed => ($this->function)($result)->{$name}(...$arguments),
        );
    }

    public function offsetExists(mixed $offset): bool
    {
        return true;
    }

    public function offsetGet(mixed $offset): mixed
    {
        return new self(
            fn(mixed $result): mixed => ($this->function)($result)[$offset],
        );
    }

    public function offsetSet(mixed $offset, mixed $value): void
    {
        throw new BadMethodCallException();
    }

    public function offsetUnset(mixed $offset): void
    {
        throw new BadMethodCallException();
    }
}

const it = new It();

final readonly class SomeClass
{
    public function __construct(
        public string $prop = '',
    ) {}

    public function method(mixed ...$args): array
    {
        return $args;
    }
}

assert(
    array_map(
        it['key'],
        [
            ['key' => 'a'],
            ['key' => 'b'],
            ['key' => 'c'],
        ],
    ) === ['a', 'b', 'c'],
    'Test 1 failed.',
);

assert(
    array_map(
        it->prop,
        [
            new SomeClass('a'),
            new SomeClass('b'),
            new SomeClass('c'),
        ],
    ) === ['a', 'b', 'c'],
    'Test 2 failed.',
);

assert(
    array_map(
        it->method('a', arg2: 'b'),
        [
            new SomeClass(),
        ],
    ) === [['a', 'arg2' => 'b']],
    'Test 3 failed.',
);

assert(
    array_map(
        it->method(key: ['key' => 'value'])['key']['key'],
        [
            new SomeClass(),
        ],
    ) === ['value'],
    'Test 4 failed.',
);