In Ruby, the convention is usually that things are duck-typed (the actual types of your inputs don't matter as long as they implement whatever you're expecting of them, if not, we throw an exception). Type hinting could be possible, but it basically runs contrary to the idea.
Now, Ruby on Rails developers are expecting some kind of magic conversion happening at the interfaces. For example, ActiveRecord maps the database datatypes to Ruby classes and will perform automated conversions on, say, date/time values. But from the developer perspective it doesn't generally matter how this conversion actually happens, as long as there's something between the layers to do the thing.