import qualified Data.Aeson as J
import qualified Data.ByteString as S
import qualified Data.ByteString.Lazy as L
import System.Environment (getArgs)

import qualified Data.Yaml as Y

main :: IO ()
main = do
    args <- getArgs
    (input, output) <- case args ++ replicate (2 - length args) "-" of
        [i, o] -> return (i, o)
        _ -> fail "Usage: json2yaml [in] [out]"
    mval <- fmap J.decode $
        case input of
            "-" -> L.getContents
            _ -> L.readFile input
    case mval of
        Nothing -> error "Invalid input JSON"
        Just val -> case output of
            "-" -> S.putStr $ Y.encode (val :: Y.Value)
            _ -> Y.encodeFile output val