Passing an array for setting variable

Question

I often see this idiom when reading php code:

public function __construct($config)
{
    if (array_key_exists('options', $config)) {
       ...
    }
    if (array_key_exists('driver_options', $config)) {
        ...
    }
}

Here I am concern with the way the parameter is used. If I were in lisp I would do:

(defun ct (&key options driver_options) 
       (do-something-with-option-and-driver_option))

But since I am in PHP I would rather have a constructor that take a list of parameter and let them be null if there a not require.

So what do you guys think about having an array as parameter in other to do some initialization-or-whatever?

In other to answer you have to take in account the point of view of the user of the function and the designer of the API.

Answer 1

Personally, I dislike that idiom. I prefer to have a long parameter list instead, if necessary.

The problem is that I can't know the elements the array can't take by looking at the function signature. On top of that, the implementations almost never check if there's any key that's not recognized, so if I mispell an array key, I get no warning.

A better alternative would be passing a configuration object. At least, there the IDE can provide me hints on the available configuration objects and the calculated default values for missing options can be moved away from the constructor you show to the getters in the configuration object. The obvious alternative is to provide setters for the several configuration options; though this doesn't help for the required ones for each no default can be provided.

Answer 2

I very much like the design pattern of "options arrays". If PHP supported Python's argument expansion, then I would agree to go with a long parameter list. But I just find foo(1, 2, 'something', true, 23, array(4), $bar); to be REALLY un-readable. I typically will use arrays when there are more than about 3 or 4 parameters that need to be set...

What I would suggest to "clean up" the constructor, is create a protected method for accessing config vars (preferably in a base class):

abstract class Configurable {
    protected $options = array();
    protected $requiredOptions = array();

    public function __construct(array $options = array()) {
        $this->options = $options;
        foreach ($this->requiredOptions as $option) {
            if (!isset($this->options[$option])) {
                throw new InvalidArgumentException('Required argument [$'.$option.'] was not set');
            }
        }
    }

    protected function _getOption($key, $default = null) {
        return isset($this->options[$key]) ? $this->options[$key] : $default;
    }
}

Then, in your class, you can overload the requireOptions array to define things that need to be set

class Foo extends Configurable {
    protected $requiredOptions = array(
        'db',
        'foo',
    );

    public function __construct(array $options = array()) {
        parent::__construct($options);
        if ($this->_getOption('bar', false)) {
            //Do Something
        }
    }
}

One thing. If you do this, PLEASE document the options required. It will make life a lot easier for those that follow you.

Answer 3

I find using arrays as parameters to be helpful when there are a lot of optional parameters. Usually I'll use an array_merge to merge the passed array with the "defaults" array. No checking required. If you have required parameters, you can use array_diff_key to determine if any required parameters are missing.

function params($p_array) {
    static $default_vals = array('p1'=>1, 'p2'=>null, 'p3'=>'xyz');
    static $rqd_params = array('p1'=>null, 'p3'=>null);
    // check for missing required params
    $missing_params = array_diff_key($rqd_params, $p_array);
    if ( count($missing_params)>0 ) {
       //return an error (i.e. missing fields)
       return array_keys($missing_params);
    }
    // Merge passed params and override defaults
    $p_array = array_merge($default_vals, $p_array);
}