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

[Martin Tomko]

whoa, that's a cool suggestion, thanks!
I finally understand the meaning of importing only some functions 
explicitly.
Very cool.

thanks
Martin

On 9/21/2010 4:18 PM, Daniel Fischer wrote:
> On Tuesday 21 September 2010 15:50:46, Martin Tomko wrote:
>    
>> 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.!   :))).
>>      
> Too ugly?
>
>    
>> How to go about it?
>>      
> You'd use
>
> myFunct a b M.! c
>
> if you have
>
> import qualified Data.Map as M
>
> at the top of your module.
> However, qualified operators are often ugly, so I suggest
>
> import qualified Data.Map as M
> import Data.Map (Map, (!))
> -- to get the type name and the (!) operator for unqualified use
>
> myFunct a b ! c
>
> or, with parentheses,
>
> (myFunct a b) ! c
>
>
>