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
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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