Type driven generic aeson instance customisation


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This package provides a newtype wrapper where you can customise aeson's generic methods using a type-level interface, which synergises well with DerivingVia.

{-# LANGUAGE DerivingVia, DataKinds, DeriveGeneric #-}
import Data.Aeson
import Deriving.Aeson
import qualified Data.ByteString.Lazy.Char8 as BL

data User = User
  { userId :: Int
  , userName :: String
  , userAPIToken :: Maybe String
  } deriving Generic
  deriving (FromJSON, ToJSON)
  via CustomJSON '[OmitNothingFields, FieldLabelModifier '[StripPrefix "user", CamelToSnake]] User

testData :: [User]
testData = [User 42 "Alice" Nothing, User 43 "Bob" (Just "xyz")]

main = BL.putStrLn $ encode testData
-- [{"name":"Alice","id":42},{"api_token":"xyz","name":"Bob","id":43}]

Deriving.Aeson.Stock contains some aliases for even less boilerplates.

  • Prefixed str = CustomJSON '[FieldLabelModifier (StripPrefix str)]
  • PrefixedSnake str = CustomJSON '[FieldLabelModifier (StripPrefix str, CamelToSnake)]
  • Snake = CustomJSON '[FieldLabelModifier '[StripPrefix str, CamelToSnake]]
  • Vanilla = CustomJSON '[]

How it works

The wrapper type has a phantom type parameter t, a type-level builder of an Option. Type-level primitives are reduced to one Option by the AesonOptions class.

newtype CustomJSON t a = CustomJSON { unCustomJSON :: a }

class AesonOptions xs where
  aesonOptions :: Options

instance AesonOptions xs => AesonOptions (OmitNothingFields ': xs) where
  aesonOptions = (aesonOptions @xs) { omitNothingFields = True }


You can use any (static) function for name modification by adding an instance of StringModifier.

data ToLower
instance StringModifier ToLower where
  getStringModifier "" = ""
  getStringModifier (c : xs) = toLower c : xs

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