[TYPO3-english] How Use the debug() function in itemsProcFunc script

[Rocky Ou]

Hey,

I am using TYPO3 4.5.11. I want to know what is in $params array so that 
I could generate different select list depend on another field's value. 
I have below TCA config:
......
'type' => 'select',
'items' => array(),
'itemsProcFunc' => 'tx_myext_element_types->main',
....

Below is my tx_myext_element_types code:
....
class tx_myext_element_types{
     function main(&$params, &$pObj){
         $params['items'] = array(
             array('One', 0),
             array('Two', 1)
         );
         t3lib_div::debug($params);
     }
}
....

The problem is there is not debug output for $params, when I edit the 
records in list Module. If I do not comment out the debug function, the 
TYPO3 does not show the form for new record.

Can any of you tell me how to use the debug() function in this kind of 
userFunc script please?

Thanks!
Rocky