Looking for advice on modelling a 3-way merge (with varying strategies)

Hello,

I'm currently trying to implement a way 3 way merge, that is dispatched
on type. Essentially, I want to be able to say "I have this large data
type which is a product of various other types - here's how to merge
it".

By 3 way merge, I mean that I have a 'new' value, a 'current' value and
a 'common ancestor' value. So far I have:

  import Control.Applicative
  import Data.Monoid

  newtype Merge a = Merge { runMerge :: Either [String] a }

  instance Functor Merge where
    fmap f a = Merge $ fmap f $ runMerge a

  instance Applicative Merge where
    pure = Merge . Right
    (Merge (Right f)) <*> (Merge (Right a)) = Merge $ Right $ f a
    (Merge (Right _)) <*> (Merge (Left c))  = Merge $ Left $ c
    (Merge (Left c)) <*> (Merge (Left c'))  = Merge $ Left $ c <> c'
    (Merge (Left c)) <*> _                  = Merge $ Left $ c

  ok :: a -> Merge a
  ok = pure

  failMerge :: String -> Merge a
  failMerge x = Merge $ Left [x]