===== Maybe =====
==== Haskell type ====
<code/Haskell> 
data  Maybe a  =  Nothing | Just a
  deriving (Eq, Ord)

instance  Monad Maybe  where
    (Just x) >>= k      = k x
    Nothing  >>= _      = Nothing

    return              = Just
    fail _              = Nothing

-- utility

   (Just _) >>  k      = k
   Nothing  >>  _      = Nothing
    
  instance  Functor Maybe  where
    fmap _ Nothing       = Nothing
    fmap f (Just a)      = Just (f a)
</code>

==== Discussion ====
=== Reference ===
hackage: [[https://hackage.haskell.org/package/base-4.6.0.1/docs/src/Data-Maybe.html#Maybe|Source]]
==== Parents ====
=== Element of ===
[[Monad . Haskell]]