Not very sure if this is what you were asking about but yes, you can write functions that are polymorphic in their effect. E.g. the map function takes a function that has some effect e and the map function itself has the same effect e
Not as far as I can tell, but it seems to let you supply handlers to discharge effects at any point. That should at least allow you to 'embed' any effect type in a more general effect type (but the 'embedding' itself would be hardcoded, so not 'first class'... I think.).
Not very sure if this is what you were asking about but yes, you can write functions that are polymorphic in their effect. E.g. the map function takes a function that has some effect e and the map function itself has the same effect e