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

[Daniel Fischer]

On Tuesday 21 September 2010 15:05:19, Martin Tomko wrote:
> 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

Note: this is a special case of the more general `fmap'. If you use fmap 
instead of mapValue, a) you don't need to write the function yourself (it's 
available from the Prelude) and b) it works unchanged if you find that 
instead of Maybe, e.g. a list is more appropriate.

>
> 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?

No, not a dirty hack. But you have to distinguish between the cases where 
your lookup was successful and those where it wasn't.
Depending on the surrounding code, for example

.... case Data.Map.lookup key map of
        Nothing -> stuff for absent key
        Just val -> stuff with val

is a good way. If you *know* that the key you want to look up is in the 
map, you can also use (!) to get the value.

>
> 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...

maybe :: b -> (a -> b) -> Maybe a -> b
maybe default transform valueOrNot

if valueOrNot is a value (Just val), the result is (transform val), if 
valueOrNot is Nothing, the result is the default.

There's a less general variant (to be imported from Data.Maybe)

fromMaybe :: a -> Maybe a -> a
fromMaybe default valueOrNot

returns val in case valueOrNot is (Just val) and the default in case of 
Nothing.

If you use that, you might as well directly use Data.Map.findWithDefault.

These are good if there's a sensible default for absent keys, otherwise the 
won't help you either.

>
> Thanks
> Martin