Data.Generic.HKD

Documentation

module Data.Generic.HKD.Types

module Data.Generic.HKD.Labels

module Data.Generic.HKD.Construction

module Data.Generic.HKD.Build

position :: forall index f structure inner. HasPosition' index (HKD structure f) (f inner) => Lens' (HKD structure f) (f inner)

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
...

field :: forall field f structure inner. HasField' field (HKD structure f) (f inner) => Lens' (HKD structure f) (f inner)

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'.
...