Alternate solution for store credit

store-credit2.hs

import Data.List

type Input = [String]

data Unsolved = Unsolved { unsolvedNumber :: Int, input :: Input }
data Solved = Solved { solvedNumber :: Int, answer :: String }

data Item = Item { index :: Int, price :: Int }

instance Show Solved where
    show (Solved n ans) = "Case #" ++ (show n) ++ ": " ++ ans

instance Eq Item where
    (==) item1 item2 = (price item1) == (price item2)
    (/=) item1 item2 = (price item1) /= (price item2)

instance Ord Item where
    compare item1 item2
        | (price item1) < (price item2) = LT
        | (price item1) > (price item2) = GT
        | otherwise = EQ
    (<)  item1 item2 = (price item1) < (price item2)
    (>=) item1 item2 = (price item1) >= (price item2)
    (>)  item1 item2 = (price item1) > (price item2)
    (<=) item1 item2 = (price item1) <= (price item2)
    max  item1 item2 = if (price item1) > (price item2) then item1 else item2
    min  item1 item2 = if (price item1) < (price item2) then item1 else item2

instance Show Item where
    show (Item index price) = show index

main :: IO ()
main = do
    input <- fmap (tail.lines) getContents
    let cases = zipWith Unsolved [1..] (splitIntoCases input)
    mapM_ putStrLn $ map (show.solve) cases

splitIntoCases :: [String] -> [Input]
splitIntoCases input
    | post == [] = [pre]
    | otherwise = pre:(splitIntoCases post)
    where (pre,post) = splitAt 3 input

solve :: Unsolved -> Solved
solve (Unsolved n input) = Solved n ans
    where (l1:l2:l3:_) = input
          credit = read l1 :: Int
          prices = map read $ words l3 :: [Int]
          items = sort $ zipWith Item [1..] prices
          list = shrink credit items
          (res1,res2) = (index $ head list,index $ last list)
          ans = (show $ min res1 res2) ++ " " ++ (show $ max res1 res2)

shrink :: Int -> [Item] -> [Item]
shrink credit items
    | sum < credit  = shrink credit $ tail items
    | sum > credit  = shrink credit $ init items
    | otherwise = items
    where sum = (price $ head items) + (price $ last items)