Haskell version of formatter code using ByteStrings instead of [Chars]

format.hs

import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as C

main :: IO ()
main = B.getContents >>=
  (mapM_ C.putStrLn) . map (C.intercalate (C.singleton ' ')) . (fmt 0 []) . (map C.words) . (C.split '\n')

fmt :: Int -> [B.ByteString] -> [[B.ByteString]] -> [[B.ByteString]]
-- Empty file
fmt _ []                    []  =                   []
-- Should not be possible to enter this state
fmt _ part                  []  =    (reverse part):[]
-- One-line file containing only whitespace
fmt _ []             ([]:   []) =                   []
-- Base case for non-newline-terminated files
fmt _ part           ([]:   []) =    (reverse part):[]
-- Base case for newline-termianted files
fmt _ part        ([]:[]:   []) =    (reverse part):[]
-- Running out of words on a line with many following blank lines
fmt _ part     ([]:[]:[]:lines) =    (reverse part):(fmt 0 [] ([]:[]:lines))
-- Print two blank lines on the second-to-last blank line
fmt _ part        ([]:[]:lines) = (reverse part):[]:(fmt 0 [] lines)
-- If I run out of words on a line, go on to the next one
fmt i part           ([]:lines) =                    fmt i part lines
-- The core recursion which builds stacks of words and yields them as lines
fmt i part ((word:words):lines) =
  if (C.length word + i + length part - 1) > 60 then
    if i == 0 then
      (word:[]):(fmt 0 [] (words:lines))
    else
      (reverse part):(fmt (C.length word) (word:[]) (words:lines))
  else
    fmt (C.length word + i) (word:part) (words:lines)