I originally wrote this article as a comment to the programming subreddit, however I’ve found myself referring to it a few times since then, and reproduce it here for posterity. It attempts to motivate monads for people coming from imperative, stateful languages (already living in IO).
In Haskell, a purely functional language, we use monads to provide a library-based approach to sequential, imperative evaluation: the famous IO monad. No wonder Haskell people talk about it a lot: library-based imperative programming in a purely functional language is fun.
But monads don’t stop there! In languages with builtin sequential evaluation the IO monad must seem boring or silly — no wonder it all seems a puzzle.
This is where it gets interesting. Since monads provide an abstraction over evaluation order, they can be used in other languages (and Haskell) to implement *non*-sequential evaluation order, as a library, also!
So instead of special language support for, say, exceptions and callcc, you just implement monads as a library, and get:
- optional results (Maybe)
- encapsulated state (ST or State)
- alternative results (Either)
- exceptions (Error)
- continuations (Cont)
- backtracking and non-deterministic evaluation
- sofware transactional memory
- and then on to Arrow-based evaluation graphs
- or, say, monadic region memory management
- even logic programming
- or the Pi calculus
Perhaps this gives a flavour for non-Haskell people for why they’re so useful. Continuations as a library! Non-deterministic evaluation as a library!
So just as its well known that with continuations you can implement threaded control or exceptions as a library, with monads you can implement continuations themselves as a library, along with all the other fun toys. Then, using monad transformers, you can compose separate monads, providing, say, exception handling over a state encapsulation: you can specify precisely what programming language concepts a particular program requires. Here is:
Users of other languages are likely, and rightly so, to be less interested in the solution to IO based on monads, and far far more interested in things like library-based continuations or exceptions (see for example monadic delimited continuations in OCaml).
Hope that gives a hint towards what this monad stuff is all about.