Hi all:

I am a newbie of haskell and I'm reading "All about Monads" right now, I have some question about the example shows in Chapter 2, it says that following code is ugly

mothersPaternalGrandfather :: Sheep -> Maybe Sheep
mothersPaternalGrandfather s = case (mother s) of
                                 Nothing -> Nothing
                                 Just m  -> case (father m) of
                                              Nothing -> Nothing
                                              Just gf -> father gf

and after intruduce the comb function, it become more cleaner:

-- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x -- now we can use `comb` to build complicated sequences mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father

my question is, why not define the function father and mother as type of father::Maybe Sheep -> Maybe Sheep? this can also

clean the code and it avoid the additional function comb

further more, why we need a function => with type of => m a -> ( a -> m b ) -> mb? define some function with type m a -> m b can solve these problems too.

Thanks and BR