Moodle how to find out the role of the logged in user

How to get context/role of logged in user in moodle? I am trying to implement a context-aware block. The block would suggest the right quizzes to its users based on their moods. Role can be a teacher, student, teacher assistant or admin. I have already found the `get_context_instance()` & `has_compatibility()` functions, but I don't know how to use them for this purpose.