monad transformer example.hs

import Control.Monad
import Control.Monad.Trans
import Control.Applicative

data EitherMaybeT e a = EitherMaybeT {runEitherMaybe :: Either e (Maybe a)}

instance Functor (EitherMaybeT e) where
  fmap f = EitherMaybeT . fmap (fmap f) . runEitherMaybe

instance Applicative (EitherMaybeT e) where
  pure    = return
  f <*> x = EitherMaybeT $ liftA2 (<*>) (runEitherMaybe f) (runEitherMaybe x)

instance Monad (EitherMaybeT e) where
  return = (\x -> EitherMaybeT $ Right (Just x))
  x >>= f =
    EitherMaybeT $
    (case runEitherMaybe x of
       Right (Just z) -> runEitherMaybe $ f z
       Right (Nothing) -> Right Nothing
       Left z -> Left z
    )

is0 :: Int -> Maybe Int
is0 x = case x of
  0 -> Nothing
  x -> Just x

isMultipleOf5 :: Int -> Either String Int
isMultipleOf5 x = case mod x 5 of
  0 -> Right x
  _ -> Left "Value was not multiple of 5"

liftMaybe :: Maybe a -> EitherMaybeT e a
liftMaybe = EitherMaybeT . Right

liftEither :: Either e a -> EitherMaybeT e a
liftEither x = case x of
  Right x' -> EitherMaybeT . Right . Just $ x'
  Left x' -> EitherMaybeT . Left $ x'

if5EitherMaybe :: Int -> EitherMaybeT String Float
if5EitherMaybe x = do
  x' <- liftMaybe $ is0 x
  x'' <- liftEither $ isMultipleOf5 x'
  return ((fromIntegral x'') / 5.0)


if5EitherMaybeNotTransformed :: Int -> EitherMaybeT String Float
if5EitherMaybeNotTransformed x = do
  case (is0 x) of
    Just x' -> case (isMultipleOf5 x) of
      Right x'' -> liftEither $ Right $ fromIntegral x'' / 5.0
      Left x'' -> liftEither $ Left x''
    Nothing -> liftMaybe $ Nothing

main :: IO ()
main = do
  print "Transformer:"
  print . runEitherMaybe $ if5EitherMaybe 0
  print . runEitherMaybe $ if5EitherMaybe 5
  print . runEitherMaybe $ if5EitherMaybe 100
  print . runEitherMaybe $ if5EitherMaybe 3
  putStrLn ""
  print "Non transformer:"
  print . runEitherMaybe $ if5EitherMaybeNotTransformed 0
  print . runEitherMaybe $ if5EitherMaybeNotTransformed 5
  print . runEitherMaybe $ if5EitherMaybeNotTransformed 100
  print . runEitherMaybe $ if5EitherMaybeNotTransformed 3

Output:

"Transformer:"
Right Nothing
Right (Just 1.0)
Right (Just 20.0)
Left "Value was not multiple of 5"

"Non transformer:"
Right Nothing
Right (Just 1.0)
Right (Just 20.0)
Left "Value was not multiple of 5"