Haskell version of "Four MLs (and a Python)"

haskell.hs

import Control.Applicative
import System.Environment
import Data.List.Split
import Data.List

-- the ":: [Float]" prevents the type inferencer from going haywire
addTo [] v = v :: [Float]
addTo t v
  | length t == length v = zipWith (+) t v
  | otherwise            = error "Inconsistent-length rows"

addLineTo t l = addTo t . map read $ splitOn "," l

sumData = foldl addLineTo [] . lines

sumAndPrintFile f = intercalate "," <$> map show <$> sumData <$> readFile f >>= putStrLn

parseArgs [f] = sumAndPrintFile f
parseArgs _   = error "Exactly 1 filename must be given"

main = getArgs >>= parseArgs