docs: function(cache) {
    // Init a promise.
    return Promise.resolve()
    .then(function() {
  
      if (!_.isEmpty(appCache)) {
        // Return cached value.
        return appCache[cache] || [];
      } else {
        // Reload and cache app dirs, then get app dir from cache again.
        return list()
        .then(function() {
          return appCache[cache] || [];
        });
      }
  
    });
  }