On 06/19/2015 01:51 AM, Matt Williams wrote:
In an imperative language this might look like:
myMap = new map() for elem in myElems: key = makeKey(elem) myMap[key] = myMap[key] + elem
...
My thought was that I needed to go through the list of the elems, and at each point add them to the Hash Map, handing the updated Map onto the next step - but this is what I cannot write.
Your thought was right. You want to go through the list of elems, building up a new value (the hash map) as you go. The pattern is called a fold, as others have mentioned. The only tricky part is gluing together the pieces. Your pseudocode above looks like it assumes that myMap[key] will return zero if `key` isn't present in `myMap`. I think I've managed to reproduce what you want. The "key from element" function I used is just the identity function, but you should be able to adapt it. module Main where import Data.Map (Map, empty, insert) import qualified Data.Map as M (lookup) key_from_elem :: Int -> Int key_from_elem = id loop :: [Int] -> (Map Int Int) -> (Map Int Int) loop elems initial_map = foldr update_map initial_map elems where update_map :: Int -> (Map Int Int) -> (Map Int Int) update_map x m = let k = key_from_elem x in case (M.lookup k m) of Nothing -> insert k x m Just v -> insert k (v + x) m main :: IO () main = do let elems = [1,2,3,4,5] let l1 = loop elems empty print l1 let l2 = loop elems l1 print l2 let l3 = loop elems l2 print l3