Documentation

Higher-kinded data (HKD) is the design pattern in which every field in our type is wrapped in some functor f:

  data User f
   = User
       { name :: f String
       , age  :: f Int
       }

Depending on the functor, we can get different behaviours: with Maybe, we get a partial structure; with Validation, we get a piecemeal validator; and so on. The HKD newtype allows us to lift any type into an HKD-style API via its generic representation.

>>> :set -XDeriveGeneric -XTypeApplications
>>> :{
data User
  = User { name :: String, age :: Int }
  deriving Generic
:}

The HKD type is indexed by our choice of functor and the structure we're lifting. In other words, we can define a synonym for our behaviour:

>>> import Data.Monoid (Last (..))
>>> type Partial a = HKD a Last

... and then we're ready to go!

>>> mempty @(Partial User)
User {name = Last {getLast = Nothing}, age = Last {getLast = Nothing}}

>>> mempty @(HKD (Int, Bool) [])
(,) [] []

Calculate the "partial representation" of a type.

Calculate the "partial representation" of a generic rep.

Often, we can get instances by using an HKD type's isomorphism with a certain size of tuple. This class witnesses the isomorphism with a certain tuple (specifically a nested tree of pairs) to allow us to derive "via" these shapes.