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).

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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)
• concurrency
• 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:

• Some examples
• Some theory
• Some code

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.