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

What you'll want to do is try to implement the instance for Functor and check if it conforms to the Functor Laws. Start with the instance:

instance Functor (Promise f) where
    fmap f (PendingPromise g) = PendingPromise g
    fmap f (ResolvedPromise a) = ResolvedPromise (f a)
    fmap f BrokenPromise = BrokenPromise

Does this conform to the Functor Laws? Since the cases for PendingPromise and BrokenPromise are always the identity, I'll exclude them for brevity:

-- fmap id = id
fmap id (ResolvedPromise a) = ResolvedPromise (id a) = ResolvedPromise a

-- fmap (f . g) = fmap f . fmap g
fmap (f . g) (ResolvedPromise a) = ResolvedPromise (f (g a))
fmap f (fmap g (ResolvedPromise a)) = fmap f (ResolvedPromise (g a)) = ResolvedPromise (f (g a))

So yes, it does conform to the Functor laws.

Next, see if you can write the Applicative and Monad instances and prove that they conform to the respective laws. If you can write an instance that does, then your data type is a Monad, irrespective of the Future class.