Is this a Functor or Monad?

Question

I implemented my own Promise structure in C# and wanted to test the concept in Haskell so after some severe brain workouts (still very new to this) I produced

Answer 1

Yes, it is a monad. The easiest way to see this is observing Promise f a ≅ Maybe (Either f a), thus it's also isomorphic to the transformer equivalent which has an already-proven standard monad instance.

type Promise' f = ErrorT f Maybe

promise2Trafo :: Promise f a -> Promise' f a
promise2Trafo (PendingPromise f) = ErrorT . Just $ Left f
promise2Trafo (ResolvedPromise a) = ErrorT . Just $ Right a
promise2Trafo BrokenPromise = ErrorT Nothing

trafo2Promise :: Promise' f a -> Promise f a
trafo2Promise = ... -- straightforward inverse of `promise2Trafo`

instance Applicative Promise where
  pure = trafo2Promise . pure
  fp <*> xp = trafo2Promise $ promise2Trafo fp <*> promise2Trafo xp

and so on.