View source for Programming performance/Christoph Haskell

* '''Language''': Haskell
* '''Skill''': Intermediate. I learned Haskell some months ago but didn't use it for a serious project so far.
* '''Time''': 30 minutes

== Code ==
<haskell>
module Main where

data Values = Values { money, lastCosts :: Double
                     , stock            :: [(Double, Double)] }

readClosingCosts :: String -> Double
readClosingCosts = read . last . words

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

readCosts :: IO [Double]
readCosts = readFile "gspc.txt" >>=
  (return . map readClosingCosts . filter notComment . lines)

buy :: Values -> Double -> Double -> Values
buy v c amount = Values { money = (1 - amount) * money v
                        , stock = (amount * (money v) / c, c) : stock v
                        , lastCosts = c }

sellAll costs (bought, _) = bought * costs

sellFinally :: Double -> Values -> Double
sellFinally costs v = money v + (sum $ map (sellAll costs) (stock v))

tooMuchUp costs (bought, origCosts) = costs >= 1.06 * origCosts

step1 costs v
    | costs <= 0.97 * lastCosts v = buy v costs 0.1
    | otherwise                   = v { lastCosts = costs }

step2 costs v = v { money = money v + newMoney
                  , stock = filter (not . tooMuchUp costs) (stock v) }
    where newMoney = sum $ map (sellAll costs)
                               (filter (tooMuchUp costs) (stock v))

step :: Double -> Values -> Values
step costs v = step2 costs (step1 costs v)

main = do
  costs <- readCosts
  print $ sellFinally (head costs) $ foldr step (Values 10000 0 []) costs
</haskell>