[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 09:37:35 EDT 2010
Hi edgar,
the whole of my code and the input data are one contained world, I am
not using mondas - as I don't understand them properly, and they seem
not to be necessary for the simple algebra I am trying to develop. Do I
really need to use them here?
Now, to your example:
>>>Try for example in ghci: fromJust $ Just 1 >>= (\x -> return $ x + 1)
(I also had to search for definition of the $ operator, totally avoided
in the two books I have, and seems to be just syntactic sugar instead of
parentheses. Argh.)
this seems to be equivalent to fromJust (Just 1), where I would assuem a
result of 1. But the example seems to be dependent on whatever x is
entered by keyboard. Am I right?
Cheers
Martin
On 9/21/2010 3:19 PM, edgar klerks wrote:
> Also note that Maybe is a monad. So you don't have to use pattern
> matching.
>
> You have to import the Data.Maybe to get the functions in the scope.
>
> On Tue, Sep 21, 2010 at 3:15 PM, edgar klerks <edgar.klerks at gmail.com
> <mailto:edgar.klerks at gmail.com>> 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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