[Haskell-beginners] help using Data.Map lookup - how to get values
after the "Just" constructor
Martin Tomko
martin.tomko at geo.uzh.ch
Tue Sep 21 09:28:49 EDT 2010
Thank you Edgar,
that helped a lot. I successfully implemented the fromJust option, I am
not sure how to define the fromMaybe one, which seems superior. I would
need two parameters (a and Maybe a, is that correct) but not sure how to
get them.
Any link to understand this better is appreciated.
Apologies for being such a hopeless newbie.
Thanks
Martin
On 9/21/2010 3:15 PM, edgar klerks wrote:
> 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 at geo.uzh.ch
> <mailto:martin.tomko at 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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