php-pslam-functional-generators.php

<?php
declare(strict_types=1);

/**
 * @psalm-template TKey
 * @psalm-template TValue
 * @psalm-param iterable<TKey, TValue> $list
 * @psalm-param callable(TValue): bool $filter
 * @psalm-return Generator<TKey, TValue>
 */
function iterable_filter(iterable $list, callable $filter): Generator
{
    foreach ($list as $k => $v) {
        if ($filter($v)) {
            yield $k => $v;
        }
    }
}

/**
 * @psalm-template TKey
 * @psalm-template TValue
 * @psalm-template TCallableReturn
 * @psalm-param iterable<TKey, TValue> $list
 * @psalm-param callable(TKey, TValue): TCallableReturn $operation
 * @psalm-return Generator<TKey,TCallableReturn>
 */
function iterable_map(iterable $list, callable $operation): Generator
{
    foreach ($list as $k => $v) {
        yield $k => $operation($k, $v);
    }
}

/**
 * @psalm-template TKey
 * @psalm-template TValue
 * @psalm-param iterable<TKey, TValue> $collection
 * @psalm-param callable(TValue, TKey): string $callback
 * @psalm-return Generator<TKey, TValue>
 */
function iterable_unique(iterable $collection, callable $callback): Generator
{
    $keys = [];
    foreach ($collection as $key => $element) {
        $index = $callback($element, $key);
        if (!array_key_exists($index, $keys)) {
            $keys[$index] = null;
            yield $key => $element;
        }
    }
}

$list = [
    'faz' => new \DateTime('tomorrow'),
    'foo' => new \DateTime('now'),
    'baz' => new \DateTime('1989-01-01'),
    'bar' => new \DateTime('tomorrow'),
    'bafr' => new \DateTime('next week'),
];

$list = iterable_unique($list, static function(\DateTime $value) : string {
    return $value->format('Ymd');
});

$list = iterable_filter($list, static function (\DateTime $value) : bool {
  return $value->format('Y') > 2000;
});

$list = iterable_map($list, static function (string $_, \DateTime $value) : int{
    return (int)$value->format('Y');
});

foreach ($list as $key => $value) {
    echo "$key:$value\n";
}