-----------------------------------------
--- Example of another Monad: Maybe
import Test.QuickCheck

mum :: String -> Maybe String
mum s = lookup s [("dave","jill"),("jill","jane"),("jane","bob"),
                  ("bob","harry"),("harry", "sally")] -- for example

ggrandmum :: String -> Maybe String
ggrandmum s = case mum s of
                   Just m -> case (mum m) of 
                                Just m' -> mum m'
                                Nothing -> Nothing
                   Nothing -> Nothing
-- Now in monadic style

{- from the Prelude:
 instance Monad Maybe where
    return         = Just
    Nothing  >>= f = Nothing
    (Just x) >>= f = f x
 -}

ggrandmum' :: String -> Maybe String
ggrandmum' s = 
   do gm <- mum s
      ggm <- mum gm
      mum ggm


data Name = Name String deriving Show

instance Arbitrary Name where 
  arbitrary = elements $
      map Name ["dave", "jill", "jane", "bob"]

prop_ggg (Name s) = collect (ggrandmum s) $
    ggrandmum' s == ggrandmum s