[Haskell-beginners] help using Data.Map lookup - how to get values after the "Just" constructor

[Martin Tomko]

Dear All,
as a newbie, I am only just discovering some intricacies of haskell.
I have a Data.Map map, and am trying the lookup function to get the 
value for a given key (it is a list in my case, btw). I am struggling to 
get access to the value, as it is constructed using Just. I know that 
the question is therefore more general then the application on Map, so I 
would be glad to get a wider picture. I Checked in Real World Haskell, 
btu did nto find and answer. In Haskell the craft of... I found the 
following (p263):

mapValue :: (a->b)-> Maybe a -> Maybe b
mapValue g (Just a) = Just (g a)
mapValue g Nothing = Nothing

Which is fine, but it makes the Just constructor travel through the 
whole code, which is annoying. Is there a way out? Or would that be a 
dirty hack?

I do not quite understand the following discussion of maybe (p263-4), 
but it seems like the code suggested is able to return a value at the end...

Thanks
Martin