Machines are monads

Machines.hs

data Machine k a where
  Emit :: a -> Machine k a -> Machine k a
  Await :: k r -> (r -> Machine k a) -> Machine k a -- ignoring fallback for now, not relevant
  Halt :: Machine k a

(++) :: Machine k a -> Machine k a -> Machine k a
Halt ++ m2 = m2
(Emit h t) ++ t = Emit h (t ++ m2)
(Await req recv) ++ t = Await req (\res -> recv res ++ t)

instance Monad (Machine k) where
  return a = Emit a Halt
  Halt >>= f = Halt
  Emit h t >>= f = f h ++ (t >>= f)
  Await req recv >>= f = Await req (\res -> recv res >>= f)