Modelling a Form in Haskell

Form.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE StandaloneDeriving #-}

module Form where

import qualified Data.Aeson as JSON
import           GHC.Generics

data Form =
  Form
  { formId  :: String
  , version :: Int
  , author  :: String
  , fields  :: [Element]
  }
  deriving (Show, Eq)

data Element = Field' Field
             | MultiField' MultiField
             deriving (Eq, Generic, Show)

data TextField =
  TextField
  { _id   :: Int
  , label :: String
  }
  deriving (Eq, Generic, Show)

data DropDownField =
  DropDownField
  { _id     :: Int
  , label   :: String
  , choices :: [(Int, String)]
  }
  deriving (Eq, Generic, Show)

data Field where
  Text     :: TextField -> Field
  DropDown :: DropDownField -> Field

deriving instance Eq Field
deriving instance Generic Field
deriving instance Show Field

instance JSON.ToJSON Field
instance JSON.ToJSON TextField
instance JSON.ToJSON DropDownField

data MultiField where
  FiveW :: { what   :: TextField
           , when   :: TextField
           , _where :: TextField
           , who    :: TextField
           , which  :: TextField
           , how    :: TextField } -> MultiField