Stack repl script

repl.hs

#!/usr/bin/env stack
{- stack script
  --resolver lts-14.6
  --package aeson
  --package bytestring
-}

{-# OPTIONS_GHC -Wall #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}

module Main where

-- import qualified Conduit
-- import qualified Data.Conduit.Attoparsec as Conduit
import Data.Aeson (ToJSON, FromJSON)
import qualified Data.Aeson as Aeson
import qualified Data.ByteString as ByteString
import qualified Data.ByteString.Char8 as ByteString.Char8
import qualified Data.ByteString.Lazy.Char8 as ByteString.Lazy.Char8
import qualified System.IO

class (FromJSON i, ToJSON o) => Jsrv i o where
  jsrvEval :: i -> o

-- eitherFail :: Monad m => Either String s => m s
-- eitherFail (Left s) = fail s
-- eitherFail (Right s) = return s


repl :: (FromJSON i, ToJSON o) => (i -> o) -> IO ()
repl eval = do
  i' <- readStdin
  case i' of
    Left err -> do
      System.IO.hPutStrLn System.IO.stderr err
      repl eval
    Right i -> do
      let o = eval i
      ByteString.Lazy.Char8.putStrLn $ Aeson.encode o
      repl eval
  where
    readStdin :: FromJSON x => IO (Either String x)
    readStdin = do
      ln <- ByteString.getLine
      return $ Aeson.eitherDecodeStrict ln

main :: IO ()
main = repl (id :: Aeson.Value -> Aeson.Value)