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@berdario berdario/Example.hs
Created Jan 6, 2017

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{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Example where
import Control.Exception
import Control.Monad
import Control.Monad.Error.Class
import Control.Monad.Trans.Except
import Data.Void
import UnexceptionalIO
class (MonadError SomeException m) => Eff1 m where
f1 :: String -> m String
class Eff2 m where
f2 :: (MonadError e m) => String -> m String
class Eff3 m where
f3 :: String -> m String
type AppM = ExceptT SomeException UIO
appReadFile :: String -> AppM String
appReadFile = ExceptT . fromIO . readFile
instance Eff1 AppM where
f1 = appReadFile
data MockIO1a a = MockIO1a a deriving (Functor)
instance Applicative MockIO1a where
pure = MockIO1a
(<*>) (MockIO1a f) = fmap f
instance Monad MockIO1a where
(MockIO1a a) >>= f = f a
instance MonadError SomeException MockIO1a where
throwError = undefined
catchError = const
instance Eff1 MockIO1a where
f1 = return
-- MockIO1a's throwError should never happen, but still it's not nice to have to use undefined.
-- I hoped I could use Void:
-- data MockIO1b a = MockIO1b (Either Void a) deriving (Functor)
-- instance (MonadError SomeException MockIO1b) where
-- throwError = absurd -- doesn't compile.
-- Maybe it could, if I wouldn't have to specialize (MonadError e) to (MonadError SomeException)
-- catchError = const
-- instance Eff1 MockIO1b where
-- f1 = pure
instance Eff2 AppM where
f2 = appReadFile
data MockIO2 a = MockIO2 a deriving (Functor)
instance Applicative MockIO2 where
pure = MockIO2
(<*>) (MockIO2 f) = fmap f
instance Eff2 MockIO2 where
f2 = pure
instance Eff3 AppM where
f3 = appReadFile
instance Eff3 MockIO2 where
f3 = pure
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