Thank you Edgar, Daniel, these helped a lot! one more question regarding the suggestion of daniel, using ! if I KNOW that the key will occur (and as I work in a closed world, it should be the case). Out of curiosity - how does one use ! ?? let's say I have a function returning a map: myFunct a b, and I was using: M.lookup c (myFunct a b) On this, I could happily use fromJust (M.lookup c (myFunct a b)) Now, if I do (myFunct a b) ! c i get ! not in scope. ! seems to come from Data.Map and I import it as import qualified Data.Map as M I don't think I can do M.! :))). How to go about it? On 9/21/2010 3:34 PM, Daniel Fischer wrote:
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