https://www.reddit.com/r/dailyprogrammer/comments/40h9pd/20160111_challenge_249_easy_playing_the_stock/

20160116-easy.hs

import qualified Data.ByteString.Char8 as C
import Data.Maybe
import Text.Printf

readPrice = fromJust . readPrice_
  where readPrice_ s = C.readInt s >>= \(x, s1) -> 
          C.uncons s1 >>= \(c, s2) ->
          case c of
           '.' -> C.readInt s2 >>= \(y, s3) -> return (x*100+y)
           _ -> Nothing

getinputs = map readPrice . C.words

maxProfitAfter x [] = Nothing
maxProfitAfter x xs = if y <= x then Nothing else Just (x, y)
  where y = maximum xs

maxProfit [] = Nothing
maxProfit (x:[]) = Nothing
maxProfit (x:y:xs) = better (maxProfitAfter x xs) (maxProfit (y:xs))
  where better Nothing x = x
        better x Nothing = x
        better l@(Just (x1, y1)) r@(Just (x2, y2)) = 
          if (y2 - x2) > (y1 - x1) then r else l

main = C.getContents >>= putStrLn . show . maxProfit . getinputs