{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts #-}

import Control.Monad.Error
import Control.Applicative

data Excep a =  Err String | Ok a
    deriving (Eq, Show)

instance Monad Excep where
    return = Ok
    Ok a >>= f = f a
    Err a >>= _ = Err a

instance Applicative Excep where
    pure  = return
    (<*>) = ap

instance Functor Excep where
    fmap = liftM

instance Alternative Excep where
    (<|>) = mplus
    empty = mzero

instance MonadPlus Excep where
    mzero = Err "MonadPlus.mzero error."
    Err _ `mplus` _ = Err "MonadPlus.mzero error."
    _ `mplus` Err _  = Err "MonadPlus.mzero error."
    Ok a `mplus` Ok b = Ok a

instance (MonadError String) Excep where
    throwError = Err
    Err  l `catchError` h = h l
    Ok r `catchError` _ = Ok r

(?/) :: (MonadError String m) => Double -> Double -> m Double
x ?/ 0 = throwError "Division by 0."
x ?/ y = return $ x / y

example :: Double -> Double -> Excep String
example x y = action  `catchError` return where
  action = do
    q <- x ?/ y
    guard (q >=0)
    if q  > 100 then do
      {-100 <- return q-}
      throwError "Monad.fail error."
    else
      return $ show q