When I use Data.Aeson library to decode a json string into a Haskell value, I want to provide default values for Maybe fields, which will be Nothing when these fields are omitted in the json string. It's a hard work when there are lots of Maybe fields or deeply nested fields, it will also make your code hard to read and to maintain. This module provides a solution by using Higher-kinded data (HKD). See this blog for more information about HKD.

For example, if you have a Config type as follows,

data Config = Config { dbHost :: String
                     , dbPort :: Int
                     , dbName :: String
                       ...
                     }

and you want to read these configuration data from a json file when you start you application, you instantiate Data.Aeson.FromJSON for the Config,

data Config = Config { dbHost :: String
                     , dbPort :: Int
                     , dbName :: String
                       ...
                     } deriving Generic

instance FromJSON Config

and you want dbPort can be omitted in the json string, a default value will be used when it is omitted, you change String type to Maybe String,

data Config = Config { dbHost :: String
                     , dbPort :: Maybe Int
                     , dbName :: String
                       ...
                     } deriving Generic

instance FromJSON Config

and decode and use the configuration data in main function as below,

main = do
  config <- fromJust <$> decodeFileStrict "./config.json"
  let host = dbHost config
      port = fromMaybe defaultDBPort $ dbPort config
      ...
  dbConn <- connectDB host port ...
  ...

it is neither elegant nor easy to maintain when you have lots of configuration items.

By using HKD and type family, it becomes easier to maintain your code.

type family HKD f a where
  HKD Identity a = a
  HKD f        a = f a

data ConfigH f = Config { dbHost = String
                        , dbPort = HKD f String
                          ...
                        } deriving Generic

instance Default ConfigH

instance FromJSON (ConfigH Maybe)

type Config = Config Identity

instance FromJSON Config where
  parseJSON v = applyDef def $ parseJSON v
    where
      def = Config undefined 3306 ...


main = do
  -- Enable RecordWildCards extension
  Config{..} <- fromJust <$> decodeFileStrict "./config.json"
  dbConn <- connectDB dbHost dbPort ...

More Examples

>>> :set -XDeriveGeneric
>>> :set -XFlexibleInstances
>>> :set -XStandaloneDeriving
>>> import           Data.Functor.Identity
>>> import           GHC.Generics
>>> import           HKD.Default
>>> :{
data Triple f = Triple String (f Int) (f Double) deriving Generic
instance Default Triple
deriving instance Show (Triple Identity)
:}
...
>>> let def = Triple "hello" (Identity 123) pi :: Triple Identity
>>> applyDef def $ Triple "world" (Just 456) Nothing
Triple "world" (Identity 456) (Identity 3.141592653589793)

>>> :set -XDeriveGeneric
>>> :set -XStandaloneDeriving
>>> :set -XFlexibleInstances
>>> :set -XOverloadedStrings
>>> import           Data.Aeson
>>> import           Data.Functor.Identity
>>> import           GHC.Generics
>>> import           HKD.Default
>>> :{
data Name f = Name { first :: f String
                   , last_ :: f String
                   } deriving Generic
instance Default Name
deriving instance Show (Name Identity)
instance FromJSON (Name Maybe)
data Person f = Person { name :: Name f -- name is required
                       , age  :: f Int  -- age is optional (can be omitted)
                       } deriving Generic
instance Default Person
deriving instance Show (Person Identity)
instance FromJSON (Person Maybe)
instance FromJSON (Person Identity) where
  parseJSON v = applyDef def <$> parseJSON v
    where
      def = Person (Name (Identity "Jorah") (Identity "Gao")) (Identity 28)
:}
>>> decode "{\"name\": {}}" :: Maybe (Person Identity)
Just (Person {name = Name {first = Identity "Jorah", last_ = Identity "Gao"}, age = Identity 28})
>>> decode "{}" :: Maybe (Person Identity)
Nothing