Hello Martin,

You have two options. You can use fromJust :: Maybe a -> a, which is a partial function, so it can fail if the supplied value is Nothing and gives a hard to track down exception.

Or you can use fromMaybe :: a -> Maybe a -> a,  which returns a default a in case Maybe a = Nothing and a if Maybe a = Just a.

There is also a third: maybe :: b -> (a -> b) -> Maybe a -> b, which can be useful in the last step of some chain of functions. Note that fromMaybe is just (flip maybe id).

Greets,

Edgar

On Tue, Sep 21, 2010 at 3:05 PM, Martin Tomko <martin.tomko@geo.uzh.ch> 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

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

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