code: public static function createRuntime()
      {
          switch ($compileDir = getenv(self::COMPILE_DIR)) {
              case false: return new AstRuntime();
              case 'on': return new CompilerRuntime();
              default: return new CompilerRuntime($compileDir);
          }
      }
docs: Creates a JMESPath runtime based on environment variables and extensions
  available on a system.
  
  @return callable