PHP闭包Closure与array_reduce结合的一个范例

最近在研究laravel5.5的源代码，发现了其中的一段代码觉得挺有意思!

文件：vendor/laravel/framework/src/Illuminate/Pipeline/Pipeline.php

    public function then(Closure $destination)
    {
        $pipeline = array_reduce(
            array_reverse($this->pipes), $this->carry(), $this->prepareDestination($destination)
        );
        return $pipeline($this->passable);
    }

    protected function carry()
    {
        return function ($stack, $pipe) {
            return function ($passable) use ($stack, $pipe) {
                if (is_callable($pipe)) {
                    // If the pipe is an instance of a Closure, we will just call it directly but
                    // otherwise we'll resolve the pipes out of the container and call it with
                    // the appropriate method and arguments, returning the results back out.
                    return $pipe($passable, $stack);
                } elseif (! is_object($pipe)) {
                    list($name, $parameters) = $this->parsePipeString($pipe);

                    // If the pipe is a string we will parse the string and resolve the class out
                    // of the dependency injection container. We can then build a callable and
                    // execute the pipe function giving in the parameters that are required.
                    $pipe = $this->getContainer()->make($name);

                    $parameters = array_merge([$passable, $stack], $parameters);
                } else {
                    // If the pipe is already an object we'll just make a callable and pass it to
                    // the pipe as-is. There is no need to do any extra parsing and formatting
                    // since the object we're given was already a fully instantiated object.
                    $parameters = [$passable, $stack];
                }
                return method_exists($pipe, $this->method)
                                ? $pipe->{$this->method}(...$parameters)
                                : $pipe(...$parameters);
            };
        };
    }

此段代码初看上去让人很迷惑，特作以下demo分解：

//闭包与array_reduce结合测试
$a  = array( 1 ,  2 ,  3 ,  4 ,  5 );
$b  =  array_reduce ( $a ,  function($v , $w){
    return function($p) use($v, $w){
        var_dump($v);
    };
} );

if(is_callable($b)){
    $b('spring');
}

输出：

object(Closure)#5 (2) {
  ["static"]=>
  array(2) {
    ["v"]=>
    object(Closure)#4 (2) {
      ["static"]=>
      array(2) {
        ["v"]=>
        object(Closure)#3 (2) {
          ["static"]=>
          array(2) {
            ["v"]=>
            object(Closure)#2 (2) {
              ["static"]=>
              array(2) {
                ["v"]=>
                NULL
                ["w"]=>
                int(1)
              }
              ["parameter"]=>
              array(1) {
                ["$p"]=>
                string(10) "<required>"
              }
            }
            ["w"]=>
            int(2)
          }
          ["parameter"]=>
          array(1) {
            ["$p"]=>
            string(10) "<required>"
          }
        }
        ["w"]=>
        int(3)
      }
      ["parameter"]=>
      array(1) {
        ["$p"]=>
        string(10) "<required>"
      }
    }
    ["w"]=>
    int(4)
  }
  ["parameter"]=>
  array(1) {
    ["$p"]=>
    string(10) "<required>"
  }
}