[kqr]

> You could invent a total different implementation for the list monad, for the maybe monad, etc (if you respect the laws).

Could you show such a different implementation for either list or maybe?

[jjaredsimpson]

    newtype HeadList a = HeadList { getHeadList :: [a] }

    instance Monad HeadList where 
         return a = HeadList [a]
         m >>= f = HeadList $ fmap (head . getHeadList . f) (getHeadList m) 

This is a list instance that only keeps the head of function result, so it's basically just a map.

But you could imagine putting any function that returns one result. min max avg normalize, etc

[ky3]

    -- const [] for a HeadList
    quux :: a -> HeadList a
    quux = HeadList . (const [])

    *Main> HeadList "hello... no wait" >>= quux
    HeadList {getHeadList = "*** Exception: Prelude.head: empty list

Such breakage is verboten. This monad instance does not fly.