Parameters for custom Twig extension/function

I’ve noticed that the Twig Extensions don’t implement an interface. Does this mean that I can define their constructor to receive anything I want without breaking stuff all over the place?

Sorry, it’s quite late here and I’d rather get an informed answer before trying stuff which may have unintended consequences. Any points of wisdom are appreciated.

See Slim Twig Flash for an example of a Twig extension written for Slim. The Flash service provider is passed to the constructor.