Script.hs

{-|
    Use this module if you like to write simple scripts with 'String'-based
    errors, but you prefer to use 'EitherT' to handle errors rather than
    @Control.Exception@.

> import Control.Error
>
> main = runScript $ do
>     str <- scriptIO getLine
>     n   <- tryRead "Read failed" str
>     scriptIO $ print (n + 1)
-}

module Control.Error.Script (
    -- * The Script Monad
    Script,
    runScript,
    scriptIO
    ) where

import Control.Exception (try, SomeException)
import Control.Monad (liftM)
import Control.Monad.IO.Class (MonadIO(liftIO))
import Control.Monad.Trans.Either (EitherT(EitherT, runEitherT))
import Control.Error.Util (errLn)
import Data.EitherR (fmapL)
import System.Exit (exitFailure)

-- Documentation
import Control.Monad.Trans.Class (lift)
import System.IO (stderr)

-- | An 'IO' action that can fail with a 'String' error message
type Script = EitherT String IO

{-| Runs the 'Script' monad

    Prints the first error to 'stderr' and exits with 'exitFailure'
-}
runScript :: Script a -> IO a
runScript s = do
    e <- runEitherT s
    case e of
        Left  e -> do
            errLn e
            exitFailure
        Right a -> return a

{-| 'scriptIO' resembles 'lift', except it catches all exceptions and converts
    them to 'String's.

    Note that 'scriptIO' is compatible with the 'Script' monad.
-}
scriptIO :: (MonadIO m) => IO a -> EitherT String m a
scriptIO = EitherT
         . liftIO
         . liftM (fmapL show)
         . (try :: IO a -> IO (Either SomeException a))