Documentation

For any record type, we can extract the labels generically using the Const functor.

>>> import Data.Generic.HKD
>>> import Data.Functor.Identity (Identity (..))

>>> data User = User { name :: String, age :: Int } deriving Generic
>>> label @User
User {name = Const "name", age = Const "age"}

Because all HKD types are valid barbies, and we have the above mechanism for extracting field names, we can ask some pretty interesting questions.

>>> import Control.Lens
>>> import Data.Maybe (isNothing)
>>> import Data.Monoid (Last (..))
>>> import Data.Generic.HKD

Let's imagine, for example, that we're half way through filling in a user's details:

>>> data User = User { name :: String, age :: Int } deriving Generic
>>> test = mempty @(HKD User Last) & field @"name" .~ pure "Tom"

We want to send a JSON response back to the client containing the fields that have yet to be finished. All we need to do is pick the fields where the values are Last Nothing:

>>> labelsWhere (isNothing . getLast) test
["age"]