如何在PHP中使用array_filter（）进行函数式编程？

Question

Say I have an array of tags

$all_tags = array('A', 'B', 'C');

And I want to create a set of URLs with $_GET variables.
I'd like the links to be:
'A' linking to "index.php?x[]=B&x[]=C"
'B' linking to "index.php?x[]=A&x[]=C"
etc. ($_GET is an array with all elements except for "current" element) (I know there's an easier way to implement this: I'm actually simplifying a more complex situation)

I'd like to use array_filter() to solve this.
Here's my attempt:

function make_get ($tag) { return 'x[]=' . $tag; }
function tag_to_url ($tag_name) {
   global $all_tags;

   $filta = create_function('$x', 'global $all_tags; return ($x != $tag_name);'); 
   return 'index.php?' . implode('&', array_map("make_get", array_filter($all_tags, "filta")));
}
print_r(array_map("", $all_tags));

But it doesn't work. I have a suspicion that maybe it has to do with how maps and filters in PHP actually mutate the data structure themselves, and return a boolean, instead of using a functional style, where they don't mutate and return a new list.

I am also interested in other ways to make this code more succinct.

Answer 1

Here's an alternative approach:

// The meat of the matter
function get_link($array, $tag) {
    $parts = array_reduce($array, function($result, $item) use($tag)
                          {
                              if($item != $tag) $result[] = 'x[]='.$tag;
                              return $result;
                          });
    return implode('&', $parts);
}

// Test driver

$all_tags = array('A', 'B', 'C');

echo get_link($all_tags, 'A');
echo "\n";
echo get_link($all_tags, 'B');
echo "\n";
echo get_link($all_tags, 'C');
echo "\n";

It's simply one call to array_reduce , and then an implode to pull the results together into a query string.

Answer 2

Based on an answer I gave in the comments ( shown here ):

<?php

  $all_tags = array('A', 'B', 'C');

  function tag_to_url($tag_name)
  {
    global $all_tags;

    $remaining_tags = array_diff($all_tags, array($tag_name));
    return sprintf('index.php?%s', 
             http_build_query(array('x'=>array_values($remaining_tags))));
  }

  echo tag_to_url('B'); // index.php?x%5B0%5D=A&x%5B1%5D=C
                        // basically: index.php?x[0]=A&x[1]=C

Basically, use array_diff to remove the entry from the array (instead of filtering), then pass it off to the http_build_query to come up with a valid URL.

Answer 3

I'm just going to answer the "why it doesnt work" part.

$filta = create_function('$x', 'global $all_tags; return ($x != $tag_name);'); 

The tag_name variable is undefined in your lambda function's scope. Now, it is defined in the creating functions scope(tag_to_url). You can't exactly use the global keyword here either, because $tag_name isn't in the global scope, it is in the local tag_to_url scope. You could declare the variable in the global scope and then it could work, but considering you're fond of functional approaches, I doubt you like global variables :)

You could do trickery with string concatenation and var_export($tag_name) and pass that to create_function() if you wanted, but there's other better ways to achieve your goals.

Also, as an aside, I'm guessing you might benefit from turning up php's error reporting level while developing. php would have thrown undefined variable notices at you, which helps debugging and understanding.

// ideally set these in php.ini instead of in the script
error_reporting(E_ALL);
ini_set('display_errors', 1);

Answer 4

Something approaching real support for functional programming styles in PHP is very, very new--it was only in PHP 5.3 that functions became first class and anonymous functions were possible.

BTW, you should never use create_function() . What it really does is define a new function in the global namespace (which will never be garbage-collected!), and it uses eval() behind the scenes.

If you have PHP 5.3 or greater, you can do this:

$all_tags = array('A', 'B', 'C');

function is_not_equal($a, $b) {
    return $a != $b;
}

function array_filter_tagname($alltags, $name) {
    $isNotEqualName = function($item) use ($name){
        return is_not_equal($item, $name);
    };
    // array_merge() is ONLY to rekey integer keys sequentially.
    // array_filter() preserves keys.
    return array_merge(array_filter($alltags, $isNotEqualName));
}

function make_url($arr) {
    return 'input.php?'.http_build_query(array('x'=>$arr));
}
$res = array_filter_tagname($all_tags, 'B');
print_r($res);
print_r(make_url($res));

If you have a PHP < 5.3, you should use a class+object for your closures instead of create_function() .

class NotEqualName {
    protected $otheritem;
    function __construct($otheritem) { // with PHP 4, use "function NotEqualName($otheritem) {"
        $this->otheritem = $otheritem;
    }
    function compare($item) {
        return $item != $this->otheritem;
    }
}

function array_filter_tagname_objectcallback($alltags, $name) {
    $isNotEqualName = new NotEqualName($name);
    return array_merge(array_filter($alltags, array($isNotEqualName,'compare')));
}

In general, however, PHP is not very well suited to a functional style, and for your particular task using array_filter() is not very idiomatic PHP. array_diff() is a better approach.