Documentation

Product types without named fields can't be addressed by field name (for very obvious reason), so we instead need to address them with their "position" index. This is a one-indexed type-applied natural:

>>> import Control.Lens ((^.))

>>> :t mempty @(HKD (Int, String) []) ^. position @1
mempty @(HKD (Int, String) []) ^. position @1 :: [Int]

As we're using the wonderful generic-lens library under the hood, we also get some beautiful error messages when things go awry:

>>> import Data.Generic.HKD.Construction
>>> deconstruct ("Hello", True) ^. position @4
...
... error:
... • The type HKD
...              ([Char], Bool) f does not contain a field at position 4
...

When we work with records, all the fields are named, and we can refer to them using these names. This class provides a lens from our HKD structure to any f-wrapped field.

>>> :set -XDataKinds -XDeriveGeneric -XTypeApplications
>>> import Control.Lens ((&), (.~))
>>> import Data.Monoid (Last)
>>> import GHC.Generics

>>> data User = User { name :: String, age :: Int } deriving (Generic, Show)
>>> type Partial a = HKD a Last

We can create an empty partial User and set its name to "Tom" (which, in this case, is pure "Tom" :: Last String):

>>> mempty @(Partial User) & field @"name" .~ pure "Tom"
User {name = Last {getLast = Just "Tom"}, age = Last {getLast = Nothing}}

Thanks to some generic-lens magic, we also get some pretty magical type errors! If we create a (complete) partial user:

>>> import Data.Generic.HKD.Construction (deconstruct)
>>> total = deconstruct @Last (User "Tom" 25)

... and then try to access a field that isn't there, we get a friendly message to point us in the right direction:

>>> total & field @"oops" .~ pure ()
...
... error:
... • The type HKD User Last does not contain a field named 'oops'.
...