Merge

Often, one finds themselves having multiple sources of knowledge for some piece of data, and having to merge these together. Perhaps we have a type representing partial information about a digital friend.

data Friend = Friend
  { name :: Maybe Text
  , email :: Maybe Text
  , age :: Max Int
  , pubKey :: PublicKey
  }

If we learn some information about a friend from someone, and some from someone else, we'll want to merge that information to have a more complete picture. That said, it might not succeed, as we may have inconsistent information like two different names or different public keys. We'll want a function of type:

f :: Friend -> Friend -> Maybe Friend

That's the pattern that this library encapsulates!

mergeFriends :: Merge [String] Friend Friend
mergeFriends =
  User
    <$> optional name   .? ["name"]
    <*> optional email  .? ["email"]
    <*> combine age     .? ["age"]
    <*> required pubKey .? ["pubKey"]

f :: Friend -> Friend -> Validation [String] Friend
f x y = runMerge mergeFriends x y

We didn't get exactly what we thought we wanted, but this Validation type is better: it is an Applicative which accumulates the errors for every field.