A Haskell Script to nicify all kinds of structured output

nicify.hs

import Data.List (replicate)

main = getContents >>= (\x -> putStr (proc 0 x))

space   = ' '
newline = '\n'

isOpening = \c -> case c of
  '[' -> True
  '{' -> True
  '(' -> True
  _   -> False

isClosing = \c -> case c of
  ']' -> True
  '}' -> True
  ')' -> True
  _   -> False

isSeparator = \c -> case c of
  ';' -> True
  ',' -> True
  _   -> False

isStringOpening = \c -> case c of
  '"' -> True
  _   -> False

indentation = flip replicate space . (2 *)

proc :: Int -> String -> String
proc indent = \s -> case s of
  x : xs
    | isOpening x ->
        let indent' = succ indent in
        newline : indentation indent ++ x : newline : indentation indent' ++ proc indent' xs 
    | isClosing x ->
        let indent' = pred indent in
        newline : indentation indent' ++ x : proc indent' xs
    | isSeparator x ->
        x : newline : indentation indent ++ proc indent (dropWhile (== space) xs)
    | isStringOpening x ->
        let (str, rest) = break (== x) xs in
        x : str ++ take 1 rest ++ proc indent (drop 1 rest)
    | otherwise ->
        x : proc indent xs
  _ -> ""