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{-# LANGUAGE OverloadedStrings #-} | |
module Main where | |
import Prelude hiding (first) | |
import Control.Applicative | |
import Data.Text (Text) | |
import qualified Data.Text as T | |
import Control.Monad.Trans.Either | |
import Control.Error ((??)) | |
import Control.Monad.Trans (liftIO) | |
import Data.Bifunctor (first) | |
data Snarf | |
= Foodly | |
| Goop | |
deriving Show | |
data Gloop | |
= Glarg | |
| Gulp | |
deriving Show | |
data GurgErr | |
= GErrOne | |
| GErrTwo | |
deriving Show | |
data Garg = GargleBlast GurgErr | |
deriving Show | |
goof :: EitherT Garg IO Snarf | |
goof = EitherT . return . | |
first (const GargleBlast GErrOne) $ Right Foodly | |
main :: IO () | |
main = do | |
e <- runEitherT $ garf =<< goof | |
case e of | |
Left err -> print e | |
Right _ -> putStrLn "FIRE ZE MISSILEZ!" | |
foo :: Snarf -> IO (Maybe Gloop) | |
foo Foodly = return $ Just Gulp | |
foo _ = return Nothing | |
garf :: Snarf -> EitherT Garg IO () | |
garf s = do | |
a <- liftIO $ foo s | |
_ <- a ?? (GargleBlast GErrTwo) | |
liftIO $ print a |
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