Here I link up up some very useful material by Robert Norris (@tpolecat) and Martin Odersky (@odersky) to introduce Monad laws and reinforce the importance of checking the laws. E.g. while Option satisfies the laws, Try is not a lawful Monad: it trades the left identity law for the bullet-proof principle.
* ERRATA *
1) on the first slide the Kleisli composition signature appears three times, but one of the occurrences is incorrect in that it is (>=>)::(a->mb)->(b->mb)->(a->mc) whereas is should be (>=>)::(a->mb)->(b->mc)->(a->mc) - thank you Jules Ivanic.