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

module Proxy where

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

instance KnownSymbol s => ToJSON (Proxy s) where
    toJSON p = object [ "type" .= symbolVal p ]

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


jsonString :: BL.ByteString
jsonString = "{\"type\": \"foo\"}"