code: public static function forMap(array $map) : callable
      {
          return function ($index) use ($map) {
              Preconditions::checkArgument(
                  array_key_exists($index, $map),
                  "The given key '%s' does not exist in the map",
                  $index
              );
              return $map[$index];
          };
      }
docs: Returns a function which performs a map lookup.
  
  @param array $map
  @return callable
  @throws \InvalidArgumentException if given a key that does not exist in the map