PossibleGen.hs

-- module Possible where
-- This example is basically the same as in Simplest.hs, only it uses
-- GHC's builtin generics instead of explicit instances of ToJSON and
-- FromJSON.

-- We enable the DeriveGeneric language extension so that GHC can
-- automatically derive the Generic class for us.

{-# LANGUAGE DeriveGeneric #-}

{-# LANGUAGE OverloadedStrings #-}


import Data.Aeson
import Data.Aeson.TH (deriveJSON, defaultOptions)
import GHC.Generics (Generic)
import Data.Possible
--  (FromJSON, ToJSON, decode, encode)
import qualified Data.ByteString.Lazy.Char8 as BL
-- import Data.Map
-- To decode or encode a value using the generic machinery, we must
-- make the type an instance of the Generic class.

data Coord = Coord { x :: Double, y :: Double
                   , z :: Possible Double}
             deriving (Show, Generic)

-- While we still have to declare our type as instances of FromJSON
-- and ToJSON, we do *not* need to provide bodies for the instances.
-- Default versions will be supplied for us.

-- $(deriveJSON defaultOptions ''Coord)

instance FromJSON Coord
instance ToJSON Coord

blen :: ToJSON a => a -> IO ()
blen a = do
  BL.putStrLn . encode $ a
  print $ toJSON a

pde = print . (\a -> decode a :: Maybe Coord)
main :: IO ()
main = do
  pde "{\"x\":3.0,\"y\":-1.0,\"z\":1}"
  pde "{\"x\":3.0,\"y\":-1.0,\"z\":null}"
  pde "{\"x\":3.0,\"y\":-1.0,\"d\":3.5e2}"

  blen $ Coord 3 (-1) (HaveData 1)
  blen $ Coord 3 (-2) HaveNull
  blen $ Coord 3 (-1) MissingData

  blen $ object ["1" .= Missing, "2" .= Null ]
  blen $ object ["a" .= Null , "b" .= Missing, "c" .= Number 1, "d" .= RawNumber "3.4e4" ]