Putting topology aside, and recognizing that 'ease' is subjective, imo Moggi's use of monads to model the denotational semantics of I/O in lazy functional languages such as Haskell is a common textbook example; the creators of Haskell had tried many solutions that did not work in practice before monads cracked it open. Even now this solution is more widely adopted than the alternatives (streaming I/O, linear I/O types, etc) and Moggi's paper remains a classic.