PHP闭包进阶与函数式组合

PHP闭包进阶与函数式组合

PHP从5.3开始支持闭包，从7.4开始支持箭头函数。今天说说闭包的进阶用法和函数式组合。

闭包可以捕获外部变量。按值捕获和按引用捕获不同。

```php
$prefix = "用户: ";
$formatName = function (string $name) use ($prefix): string {
return $prefix . $name;
};
echo $formatName("张三") . "\n";

$counter = 0;
$increment = function () use (&$counter) {
$counter++;
};
$increment();
$increment();
echo $counter . "\n";
?>

函数工厂返回闭包。

```php
function createDiscount(float $rate): Closure
{
return fn(float $price) => round($price * (1 - $rate), 2);
}

$vipDiscount = createDiscount(0.2);
$seasonalDiscount = createDiscount(0.15);
echo "VIP价: " . $vipDiscount(299) . "\n";
echo "季末价: " . $seasonalDiscount(299) . "\n";
?>

函数组合可以把多个小函数组合成复杂的处理流程。

```php
function pipe(callable ...$fns): Closure
{
return function ($value) use ($fns) {
foreach ($fns as $fn) $value = $fn($value);
return $value;
};
}

$trim = fn(string $s): string => trim($s);
$lower = fn(string $s): string => strtolower($s);
$slug = fn(string $s): string => preg_replace('/\s+/', '-', $s);
$shorten = fn(string $s): string => substr($s, 0, 50);

$makeSlug = pipe($trim, $lower, $slug, $shorten);
echo $makeSlug(" Hello World! ") . "\n";
?>

柯里化把多参数函数转换成单参数函数链。

```php
function curry(callable $fn, ...$args): Closure
{
return function (...$newArgs) use ($fn, $args) {
$all = array_merge($args, $newArgs);
$ref = new ReflectionFunction($fn);
if (count($all) >= $ref->getNumberOfRequiredParameters()) {
return $fn(...$all);
}
return curry($fn, ...$all);
};
}

function add(int $a, int $b, int $c): int
{
return $a + $b + $c;
}

$curried = curry('add');
$add5 = $curried(5);
echo $add5(10, 20) . "\n";
?>
>

闭包是PHP函数式编程的基础。理解闭包的工作原理后，写出来的代码会更灵活。闭包用在回调、事件处理、路由处理、中间件中都很常见。