write a simple field type for CCK

 Hi all,
  I'm new to drupal module development, and want to write a simple field
type for CCK, just to give a list for user to select, and then store the
value. Drupal Verison is 6.22
To make it simple, I only write some hooks that I think may needed in the
module....
and things get wrong, the row in the table is created in the database,  but
the field is not able to store data in the database, when I click submit,
the page comes to be blank.

Here's the code for zebra.module. Did I forget some thing, thanks for your
help.

<?php

function zebra_field_info() {
    return array(
        'zebra' => array('label' => 'Zebra field',),
    );
}

function zebra_field_settings($op, $field) {
    switch ($op) {
        case 'database columns':
            $columns['value'] = array(
                'type' => 'varchar',
                'length' => 255,
                'not null' => FALSE,
                'sortable' => TRUE,
            );
            return $columns;
    };
}

function zebra_field($op, &$node, $field, &$items, $teaser, $page) {
    switch ($op) {
    }
}

function zebra_widget_info() {
    return array(
        'zebra_widget' => array(
            'label' => 'Zebra Widget',
            'field types' => array('zebra'),
        ),
    );
}

function zebra_widget_settings($op, $widget) {
    switch ($op) {
    }
}

function zebra_widget($op, &$node, $field, &$items) {
    $element['value'] = array(
        '#type' => 'select',
        '#title' => t($field['widget']['label']),
        '#default_value' => variable_get('zebra_widget_value', '0'),
        '#options' => array('a', 'b', 'c', 'd'),
    );

    return $element;
}

thanks,
-- 
Zan Guo
E-Mail: [email protected]