Continuations / Promises / Futures form a Monad (https://www.stackage.org/haddock/lts-9.1/transformers-0.5.2....)

A Monad is any data structure with the follwing two methods:

    (>>=) :: m a -> ( a -> m b) -> m b
    pure :: a -> m a

the monadic bind is exactly `andThen` in javascript:

    (>>=) :: m a -> (a -> m b) -> m b
    (>>=) :: Future a -> (a -> Future b) -> Future b
    Future.andThen :: Future a -> (a -> Future b) -> Future b

and the monadic `pure` is exactly the `Future` constructor:

    pure :: a -> m a
    pure :: a -> Future a
    Future.always :: a -> Future a

Every Monad is a Functor, so you also get this one for free:

    map :: (a  -> b ) -> m a -> m b
    -- if I know how to convert a's to b's I can convert a future a to a future b
    Future.map :: (a -> b) -> Future a -> Future b

So basically, saying "futures are a monad" gives you all the useful functions that you would expect from a future based API, which is nice. It means you can use all kinds of higher level functions not specific to futures to abstract behaviour:

Like, given a list of element,s and a way to create a future for each element, create a future that returns a list of elements:

    forM :: (Monad m) => Array a -> (a -> m b) -> m (Array b
    Future.forEvery :: Array a -> (a -> Future b) -> Future (Array b)