Find the maximum profit from buying and selling once.

stocks.hs

data StockState = StockState { hi :: Int, lo :: Int, lowest :: Int }
  deriving Show

solve xs = (hi st) - (lo st)
  where
    st = solve' xs

solve' [] = StockState 0 0 0
solve' (x:xs) = foldl go (StockState 0 0 x) xs
  where
    go st price = let
        lowest' = min price (lowest st)
      in if price - (lowest st) > (hi st) - (lo st)
      then StockState price (lowest st) lowest'
      else StockState (hi st) (lo st) lowest'
      
-- examples

solve' [10, 30, 42, 15, 20, 50, 10, 25]  -->  StockState {hi = 50, lo = 10, lowest = 10}
solve [10, 30, 42, 15, 20, 50, 10, 25] -->  40

solve' [30, 42, 15, 20, 50, 10, 25]  -->  StockState {hi = 50, lo = 15, lowest = 10}
solve [30, 42, 15, 20, 50, 10, 25] -->  35

solve' [40, 30, 20, 10] -->  StockState {hi = 0, lo = 0, lowest = 10}
solve [40, 30, 20, 10] -->  0