yaml.module

<?php

/**
 * Implements hook_element_info().
 *
 * YAML element type is basically a copy of the textarea element type.
 */
function yaml_element_info() {
  $types['yaml'] = array(
    '#input' => TRUE,
    '#cols' => 60,
    '#rows' => 5,
    '#resizable' => TRUE,
    '#process' => array('ajax_process_form'),
    '#theme' => 'textarea',
    '#theme_wrappers' => array('form_element'),
    '#attributes' => array(
      'style' => 'font-family:monospace',
    ),
  );
  return $types;
}

/**
 * Implements form_type_HOOK_value().
 *
 * @param $element
 * @param bool $input
 * @param array $form_state
 * @return string
 */
function form_type_yaml_value($element, $input = FALSE) {
  try {
    if ($input === FALSE) {
      return \Symfony\Component\Yaml\Yaml::dump($element['#default_value']);
    }
    else {
      return isset($input) ? \Symfony\Component\Yaml\Yaml::parse($input) : null;
    }
  } catch (\Symfony\Component\Yaml\Exception\ParseException $e) {
    $error_field = implode('][', $element['#parents']);
    form_set_error($error_field, $e->getMessage());
    return $input;
  }
}