[thedufer]

> And a future is often called "not truly a monad."

I think you've misunderstood this observation. It is possible to write a future library that gives a monad (I use such a library regularly). But the most common ones do not, because it happens to be a decent design decision in dynamically-typed languages to not quite obey the monad equations.

The next interesting monad is probably Haskell's Either (or OCaml's Result, if you're into that). It is only a slight twist on the optional monad. Where optional's None case contains no data, Either's Left case can contain data.

After the collections (list and optional), either, and future monads, the difficulty to understand useful monads without first understanding the category of monads jumps considerably. If you're interested, the next ones to look at would be the reader, writer, state, and continuation monads. There's also the classic example from Haskell, the IO monad.