module Err where

type E a = Maybe a

unitE :: a -> E a
unitE a = Just a 

bindE :: E a -> (a -> E b) -> E b
(Just a) `bindE` k = k a 
Nothing  `bindE` k = Nothing

tryE :: E a -> E a -> E a
tryE (Just v) h = Just v
tryE Nothing  h = h

failE :: String -> E a
failE _ = Nothing