Either Monad

module Main where

{-
-- Instancias comentadas pois já estão definidas
instance Functor (Either a) where
  fmap f (Left x) = Left x
  fmap f (Right x) = Right (f x)

instance Applicative (Either a) where
  pure x = Right x

  (Left f)  <*> _         = Left f
  _         <*> (Left x)  = Left x
  (Right f) <*> (Right x) = Right (f x)

instance Monad (Either a) where
  (Left x) >>= f  = Left x
  (Right x) >>= f = f x
-}

-- Exemplo
safeDiv :: (Show a, Eq a, Floating a) => a -> a -> Either String a
safeDiv a b
  | b == 0 = Left ("Divisao por zero: " ++ show a ++ "/" ++ show b)
  | otherwise = Right (a/b)

safeLog :: (Show a, Ord a, Eq a, Floating a) => a -> Either String a
safeLog x
  | x <= 0    = Left ("Log invalido: " ++ show x)
  | otherwise = Right (log x)

safeAdd :: Floating a => a -> a -> Either String a
safeAdd a b = Right (a+b)

safeExpr1, safeExpr2 :: Either String Double
safeExpr1 = do q <- safeDiv 10 (-2)
               l <- safeLog q
               safeAdd 2 q

safeExpr2 = do q <- safeDiv 10 0
               safeAdd 2 q

main = do
  print safeExpr1
  print safeExpr2