{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}

module Proxy where

import Control.Applicative ((<$>))
import Control.Monad (mzero)
import Data.Aeson
import Data.Aeson.Types
import qualified Data.Aeson as A
import qualified Data.ByteString.Lazy as BL
import Data.Monoid ((<>))
import Data.Proxy (Proxy(Proxy))
import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
import Data.Text (pack)

-- | Instances for serializing Proxy
instance KnownSymbol s => ToJSON (Proxy s) where
    toJSON = A.String . pack . symbolVal

instance KnownSymbol s => FromJSON (Proxy s) where
    parseJSON (A.String s) | s == pack (symbolVal (Proxy :: Proxy s)) = return (Proxy :: Proxy s)
    parseJSON _      = mzero

-- | our new data type.
newtype Payload (s :: Symbol) a = Payload a

-- | ToJSON instance
instance (KnownSymbol s, ToJSON a) => ToJSON (Payload s a) where
    toJSON (Payload a) = object [ "type" .= (Proxy :: Proxy s)
                                , "data" .= a
                                ]

-- | FromJSON instance
instance (KnownSymbol s, FromJSON a) => FromJSON (Payload s a) where
    parseJSON (Object v) = (v .: "type" :: Parser (Proxy s))
                           >>
                           Payload <$> v .: "data"
    parseJSON _          = mzero

-- | Show intance for ghci
instance (KnownSymbol s, Show a) => Show (Payload s a) where
    show (Payload a) = "Payload " <> symbolVal (Proxy :: Proxy s) <> " " <> show a

jsonString :: BL.ByteString
jsonString = "{\"type\": \"String\", \"data\": \"cool\"}"