import System.Environment (getArgs)
import Text.Yaml hiding (decode)
import Text.JSON hiding (encode)
import qualified System.IO.UTF8 as U
import Control.Monad
import qualified Data.ByteString as B

instance ToObject JSValue where
    toObject JSNull = toObject ""
    toObject (JSBool True) = toObject "true"
    toObject (JSBool False) = toObject "false"
    toObject (JSString s) = toObject $ fromJSString s
    toObject (JSArray vals) = toObject vals
    toObject (JSObject o) = toObject $ fromJSObject o
    toObject (JSRational b r) = toObject $ showJSRational' b r $ ""

toMonad :: Monad m => Result t -> m t
toMonad (Ok a) = return a
toMonad (Error s) = fail s

main :: IO ()
main = do
    args <- getArgs
    when (length args > 2) $ error "Usage: json2yaml [in] [out]"
    let (input:output:_) = args ++ repeat "-"
    content <-
        case input of
            "-" -> U.getContents
            _ -> U.readFile input
    json <- toMonad $ decode content
    let obj = toObject (json :: JSValue)
    case output of
        "-" -> B.putStr $ encode obj
        _ -> encodeFile output obj