Main.hs

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs #-}
module Main where

import System.IO
import Data.Aeson
import Data.Aeson.Types
import qualified Data.ByteString.Lazy.UTF8 as BLU

class ConfigGenerator_ a where
  generate :: a -> Maybe String

data ConfigGenerator where
  ConfigGenerator :: (ConfigGenerator_ a) => a -> ConfigGenerator

newtype Vimrc = Vimrc Object
newtype TOML = TOML Object
data NoGen = NoGen 

instance ConfigGenerator_ Vimrc where
  generate (Vimrc a) = parseMaybe (.: "a") a

instance ConfigGenerator_ TOML where
  generate (TOML a) = parseMaybe (.: "a") a

instance ConfigGenerator_ NoGen where
  generate _ = Nothing

instance ConfigGenerator_ ConfigGenerator where
  generate (ConfigGenerator gen) = generate gen

getGenerator :: Object -> ConfigGenerator
getGenerator o 
  | t == Just "vimrc" = ConfigGenerator $ Vimrc o
  | t == Just "toml" = ConfigGenerator $ TOML o
  | otherwise = ConfigGenerator NoGen
    where
      t = parseMaybe (.: "type") o :: Maybe String

makeConfig :: Maybe Object -> Maybe String
makeConfig obj = obj >>= (generate . Vimrc)

main :: IO ()
main = do
  obj <- openFile "test.json" ReadMode >>= hGetContents
  print $ (decode (BLU.fromString obj) :: Maybe Object) >>= (generate . getGenerator)