advent_day_02.hs

module Main where
import Data.Maybe
import Data.List

main = do
    content <- getContents
    let boxes = map (fromJust . strToBox) $ lines content
    putStr "Paper: "
    print . sum . map boxToPaper $ boxes
    putStr "Ribbon: "
    print . sum . map boxToRibbon $ boxes
   
data Box = Box Int Int Int deriving (Show)

boxToPaper :: Box -> Int
boxToPaper (Box l w h) = 2 * a + 2 * b + 2 * c  + minimum [a,b,c]
	where a = l * w
	      b = w * h
	      c = h * l

boxToRibbon :: Box -> Int
boxToRibbon (Box l w h) = 2 * (!!) sorted 0 + 2 * (!!) sorted 1 + l * w * h
	where sorted = sort [l, w, h]

strToBox :: String -> Maybe Box
strToBox str
    | length splits == 3 = Just $ Box (read $ (!!) splits 0) (read  $ (!!) splits 1) (read $ (!!) splits 2)
    | otherwise = Nothing
    where splits = split str 'x'

split :: String -> Char  -> [String]
split xs x = foldr (splitOne x) [] xs

splitOne :: Char -> Char -> [String] -> [String]
splitOne _ x [] = [[x]]
splitOne a b (x:xs)
    | a == b = []:x:xs
    | otherwise = (b:x):xs

I like that you have a Box datatype, didn't think of introducing that. You could have avoided the excessive use of the (!!) operator by binding l, w, h to the values as early as possible. See https://github.com/ksallberg/lekstugan/blob/master/adventofcode/Day2_2.hs