convert dynamic key fields in JSON

here is my JSON structure, there are N records that has a name as ID to represent a children

{"Kids":
  {"Jack":{"age":10}
  ,"Jane":{"age":9} 
  , .......
  }
}

in the data type in Haskell

data Kid = Kid { name::String, age::Int}

instance FromJSON Kid where
  parseJSON (Object v) =
    ....

question is ,how to make the key ( name ) as part of the constructor ? the expected output signature is like:

decode "input json string" -> [Kid]

when the expect decode function was called, it will return a list of type Kid. Thanks for reading this & appreciate any help .

Solution

By using the withObject function, you get access to an Object which is actually a KeyMap which you can manipulate much like the usual Map from e.g. containers. If you're on an older aeson version, Object will instead be a HashMap, so you can use that as well.

EDIT: I remember that Map itself also has a FromJSON, so you can probably use that instead for a shorter "solution":

{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson (FromJSON(..), withObject, (.:), fromJSON)
import Data.Map (Map)
import qualified Data.Map as Map

data Kid = MkKid {name :: String, age :: Int}

newtype Kids = MkKids {unKids :: [Kid]}

instance FromJSON Kids where
  parseJSON = withObject "Kids" $ \o -> do
    kvmap <- o .: "Kids"
    pure $ MkKids $ map (uncurry MkKid) $ Map.toList kvmap

Old "solution", which manipulates they KeyMap

{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.Aeson.Key (Key)
import qualified Data.Aeson.Key as Key
import qualified Data.Aeson.KeyMap as KeyMap
import Data.Aeson.Types (Parser, Object, Value)
import Data.Aeson (FromJSON(..), withObject, (.:), fromJSON)

data Kid = MkKid {name :: String, age :: Int}

newtype Kids = MkKids {unKids :: [Kid]}

instance FromJSON Kids where
  parseJSON = withObject "Kids" $ \o -> do
    inner <- o .: "Kids"
    withObject "inner" parseKids inner
    where
      parseKids :: Object -> Parser Kids
      parseKids obj =
        fmap MkKids $ traverse toKid $ KeyMap.toList obj
      toKid :: (Key, Value) -> Parser Kid
      toKid (k, v) = do
        age <- parseJSON v
        let name = Key.toString k
        pure $ MkKid {name, age}