Programming performance/Magnus Haskell

• Language: Haskell
• Skill: Intermediate. I'm self taught in Haskell and use it as a hobbyist but haven't used it much for serious work.
• Time: 25 minutes.
• Notes: 10 minutes spent on a silly bug where I had typed 1 instead of 100. This code is not very pretty but it works. A while after submitting I noticed I used the wrong data column. It should say column 4, not 3 in closingPrice.

Code

import Data.List (partition,foldl')

closingPrice::String->Double
closingPrice line =
    read (words line !! 3)

sell assets price = sum (map (\(_,shares)->shares*price) assets)

tradeOnce (cash,assets,lastPrice) todayPrice =
    if todayPrice <= lastPrice * 97/100 then
        let buyCost = cash*10/100 in
           (cash-buyCost, (lastPrice,buyCost/lastPrice):assets, todayPrice)
    else let (toSell,toKeep) =
                 partition (\(buyPrice,_)->todayPrice >= buyPrice*106/100) assets
         in (cash + sell toSell todayPrice,
             toKeep,
             todayPrice)

notComment ('#':rest) = False
notComment _ = True

trade input =
    let (firstPrice:restPrices) = reverse $
                                  map closingPrice $
                                  filter notComment $
                                  lines input
    in foldl' tradeOnce (10000.0,[],firstPrice) restPrices

main = do input <- readFile "gspc.txt"
          let (cash,toSell,lastPrice) = trade input
          let result = cash + sell toSell lastPrice
          print result