Provides Template Haskell expressions for deriving ToJSON
and FromJSON
instances for GADTs.
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
import Data.Aeson
import Data.Aeson.GADT.TH
data A :: * -> * where
A_a :: A a
A_b :: Int -> A ()
data B c :: * -> * where
B_a :: c -> A a -> B c a
B_x :: B c x
deriveJSONGADT ''A
deriveJSONGADT ''B
main :: IO ()
main = return ()
encode A_a
> "[\"A_a\",[]]"
When decoding a JSON-encoded GADT, the result will be wrapped using Data.Some.This.
case (decode $ encode A_a) :: Maybe (Some A) of
Nothing -> error "Couldn't decode
Just (This A_a) -> putStrLn "it worked"
Just (This A_b) -> putStrLn "wat"
> it worked