The reason why people talk so much about monads is that they are ubiquitous, and Haskell in particular can make good use of them because if type classes and oth

The reason why people talk so much about monads is that they are ubiquitous, and Haskell in particular can make good use of them because if type classes and other high level abstraction facilities, plus do notation. Asynchronous execution is one particular case that I like as an example: You can implement promises (which represent async computations) in Haskell and such a Promise type will also be able to implement the monad type class. Because they are monads, you can use do notation: -- Let's assume we want to fetch two large datasets asynchronously over the network and filter them asyncSanitize :: Promise [String] -> Promise [String] -> Promise [String] asyncSanitize p1 p2 = do x <- p1 y <- p2 return (sanitize (x ++ y)) -- no callbacks! which looks like the same code you would use for filtering two Maybe values using do notation. In fact, it is the same code, and if you leave out the type signature, then "asyncSanitize" will work for any monadic value: you could easily test your function using eg. Identity [String] (pure values), IO String (synchronous fetching), or decide to switch to some Database monad instead that fetches the values on demand from the Database. The code will not need to change at all.