[Haskell-beginners] Need help to write join on my Temporals

[martin]

Hello all,

I have these Types

type Time = Integer
data Change a = Chg {
            ct :: Time, -- "change time"
            cv :: a     -- "change value"
        } deriving (Eq,Show)

data Temporal a = Temporal {
    td :: a,         -- "temporal default"
    tc :: [Change a] -- "temporal changes"
} deriving (Eq, Show)

And I am trying to make Temporal a Monad by implementing join. My first Attempt was awfully verbose. It was also faulty,
which was revealed by this QuickQueck (I am happy with my implementation of <*>)

prop_tJoin tpr1 tpr2 = let f  = (*)
                           y1 =  f <$> tpr1 <*>  tpr2
                           y2 = (f <$> tpr1) `ap` tpr2
                       in y1 == y2


While I understand why my first implementatin was faulty, I fail to come up with anything remotely readable. I don't
understand why I am having such difficulties, let alone solve them. I feel I am approaching this from the wrong angle.

Any advice would be very much appreciated.