A "side effect" is generally the behaviour that can be modeled by a Monad under chaining (join), and is not necessarily IO (although IO is a Monad). For a burri

A "side effect" is generally the behaviour that can be modeled by a Monad under chaining (join), and is not necessarily IO (although IO is a Monad). For a burrito, the side effect of join is the union of burrito contents.