Monads in the Wild

Monads are so ubiquitous in programming that most libraries (even in general-purpose programming languages) expose monads. For example, the ReactiveX library in Java, which provides facilities for reactive programming, exposes an Observable class, which is a monad. In addition, most stream processors in data streaming libraries (across many languages) are also monads. You will typically know when something is a monad if it has a method called flatMap, which is the same as >>= in Haskell.

Therefore, whenever you are defining your own libraries for your own needs, think about what behaviours your library should support:

• Does your library involve nondeterminism or streams/lists of data?
• Does your library perform I/O?
• Does your library produce potentially empty computation?
• Does your library potentially fail?
• Does your library read from an environment?
• Does your library write to additional state?
• Does your library process state?

If the answer to one (or more) of the questions above is yes, chances are, your library should expose a monad! Furthermore, if you are writing Haskell code, your library functions can likely be described as the composition of some of the commonly used monads provided in Haskell's Prelude, the transformers library, or the mtl library.

Give this a try in the exercises and the assignment!