Haskell: Handling resulting Either from computations

Question

I have revisited Haskell lateley and constructed a toy programming language parser/interpreter. Using Parsec for lexing and parsing and a separate interpreter. I'm running in to some issues with feeding the result from the parser to my interpreter and handle the potential error from both the interpreter and parser. I end up with something like this:

main = do
  fname <- getArgs
  input <- readFile (head fname)
  case lparse (head fname) input of
    Left msg -> putStrLn $ show msg
    Right p -> case intrp p of
      Left msg -> putStrLn $ show msg
      Right r -> putStrLn $ show r

This dosn't look pretty at all. My problem is that lparse returns Either ParseError [(String, Stmt)] and itrp returns the type Either ItrpError Stmt so I'm having a real hard time feeding the Right result from the parser to the interpreter and at the same time bail and print the possible ParseError or IntrpError.

The closest to what i want is something like this

main = do
  fname <- getArgs
  input <- readFile (head fname)
  let prog = lparse (head fname) input
  (putStrLn . show) (intrp <$> prog)

But this will not surprisingly yield a nested Either and not print pretty either.

So are there any nice Haskell ideomatic way of doing this threading results from one computation to another and handling errors (Lefts) in a nice way without nesting cases?

Edit

adding types of lparse and itrp

lparse :: Text.Parsec.Pos.SourceName -> String -> Either Text.Parsec.Error.ParseError [([Char], Stmt)]

intrp :: [([Char], Stmt)] -> Either IntrpError Stmt

Answer 1

While not perfect, I'd create a helper function for embedding any Showable error from Either into MonadError:

{-# LANGUAGE FlexibleContexts #-}
import Control.Monad.Except

strErr :: (MonadError String m, Show e) => Either e a -> m a
strErr = either (throwError . show) return

Then if you have a computation that can fail with errors, like

someFn :: ExceptT String IO ()
someFn = strErr (Left 42)

you can run it (printing errors to stdout) as

main :: IO ()
main = runExceptT someFn >>= either putStrLn return

In your case it'd be something like

main = either putStrLn return <=< runExceptT $ do
  fname <- liftIO getArgs
  input <- liftIO $ readFile (head fname)
  prog <- strErr $ lparse (head fname) input
  r <- strErr $ interp prog
  print r