quasi blocchi in php

08 December 2006 // php. stuff.

Every programmer knows that iterators are generally better than loops. Although php has a variety of iterator functions, their use is seriously limited, because there is no way to pass user code to the iterator.

The overly verbose create_function() is a very unequal replacement for lambdas or blocks, but in php we have no other choice. We can only try to reduce verbosity.

function qe($body) {
    static $cache = array();
    if(!isset($cache[$body])) {
        $argc = 0;
        preg_replace('/\$(\d)/e', '$argc = max($argc, $1)', $body);
        $args = '$_' . implode("='',\$_", range(0, $argc)) . "=''";
        $expr = preg_replace('/\$(\d)/', '$_$1', $body);
        $cache[$body] = create_function($args, "return $expr;");
    }
    return $cache[$body];
}

qe() (= "quoted expression") creates a dynamic function from an expression given in a string. $0...$9 act as function parameters (like in preg_replace). Examples

# find odd numbers < 30

$a = array_filter(range(1, 30), qe('$0 % 2'));
print_r($a);

# muliply two arrays

$a = array(3, 5, 7);
$b = array(4, 6, 8);

$product = array_map(qe('$0 * $1'), $a, $b);
print_r($product);

Unlike create_function, qe also caches its results, so that the identical functions are created only once. This is quite practical when calling qe in a loop.

qe requires its argument to be an expression, i.e. no operators are allowed. The slight variation called qf ("quoted function") takes the complete function body as an argument.

function qf($body) {
    static $cache = array();
    if(!isset($cache[$body])) {
        $argc = 0;
        preg_replace('/\$(\d)/e', '$argc = max($argc, $1)', $body);
        $args = '$_' . implode("='',\$_", range(0, $argc)) . "=''";
        $expr = preg_replace('/\$(\d)/', '$_$1', $body);
        $cache[$body] = create_function($args, "$expr;");
    }
    return $cache[$body];
}

Example

$apples = array(0, 1, 2, 3);

$messages = array_map(qf('
    if($0 == 0) return "no apples";
    if($0 == 1) return "one apple";
    else return "$0 apples"
'), $apples);

print_r($messages);

Of course, this kind of code only makes sense with very short function definitions.