[Haskell-beginners] List -> Map -> List, to add duplicates?

[Jonas Almström Duregård]

Is this what you want? (note that it does not preserve ordering, but neither
does Map):

import Data.List
import Data.Function

noDups :: (Ord b, Num a) => [(a,b)] -> [(a,b)]
noDups = map sumGroup . groupBy ((==) `on` snd) . sortBy (compare `on` snd)
where
  sumGroup xs = (sum $ map fst xs, snd $ head xs)

If you use the fact that the list has at most one value per currency,
perhaps you should just keep the values in a map instead.

/J

On 7 June 2011 03:43, Tom Murphy <amindfv at gmail.com> wrote:

> Hi All,
>     This seems like an inefficient way to do what I'm trying to do.
> I'd really appreciate any suggestions or comments:
>
>
> import qualified Data.Map as Map
>
> data Currency = Dollar
>              | Yen
>              | XP
>              | Health
>              | Street_Cred
>              | Peso
>    deriving (Show, Eq, Ord) -- why ord?
>
> withDups = [(30, Dollar), (-20, Street_Cred), (-2, Dollar), (30, XP),
> (15, Peso), (30, XP)]
>
>
> flipAssoc (a, b) = (b, a)
>
> noDups = Map.fromListWith (+) (map flipAssoc withDups)
>
> final = map flipAssoc $ Map.toList noDups
>
>
>
> Thanks for your time!
> Tom
>
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