Data.Generic.HKD.Labels

Documentation

class Label (structure :: Type) where

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 (deconstruct @Identity (User "Tom" 25))
User {name = Const "name", age = Const "age"}

Methods

label :: HKD structure f -> HKD structure (Const String)

Instances

(Generic structure, GLabels (Rep structure)) => Label structure

Defined in Data.Generic.HKD.Labels

Methods

label :: HKD structure f -> HKD structure (Const String)

class GLabels (rep :: Type -> Type) where

Methods

glabel :: GHKD_ f rep p -> GHKD_ (Const String) rep p

Instances

(GLabels left, GLabels right) => GLabels (left :*: right)

Defined in Data.Generic.HKD.Labels

Methods

glabel :: GHKD_ f (left :*: right) p -> GHKD_ (Const String) (left :*: right) p

GLabels inner => GLabels (D1 meta inner)

Defined in Data.Generic.HKD.Labels

Methods

glabel :: GHKD_ f (D1 meta inner) p -> GHKD_ (Const String) (D1 meta inner) p

(TypeError (Text "You can't collect labels for a non-record type!") :: Constraint) => GLabels (C1 (MetaCons name fixity False) inner)

Defined in Data.Generic.HKD.Labels

Methods

glabel :: GHKD_ f (C1 (MetaCons name fixity False) inner) p -> GHKD_ (Const String) (C1 (MetaCons name fixity False) inner) p

GLabels inner => GLabels (C1 (MetaCons name fixity True) inner)

Defined in Data.Generic.HKD.Labels

Methods

glabel :: GHKD_ f (C1 (MetaCons name fixity True) inner) p -> GHKD_ (Const String) (C1 (MetaCons name fixity True) inner) p

KnownSymbol name => GLabels (S1 (MetaSel (Just name) i d c) (K1 index inner :: Type -> Type))

Defined in Data.Generic.HKD.Labels

Methods

glabel :: GHKD_ f (S1 (MetaSel (Just name) i d c) (K1 index inner)) p -> GHKD_ (Const String) (S1 (MetaSel (Just name) i d c) (K1 index inner)) p

labelsWhere :: forall structure f. (Label structure, ProductB (HKD structure), TraversableB (HKD structure)) => (forall a. f a -> Bool) -> HKD structure f -> [String]

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"]