Json emitter in Haskell

Json.hs

-- | Naive JSON Emitter
module Json where

import Data.Map as Map
import Data.Char as Char

import Text.Printf

type NumJ = Double

data ValueJ
  = ObjectJ (Map String ValueJ)
  | ArrayJ [ValueJ]
  | StringJ String
  | NumberJ NumJ
  | BoolJ Bool
  | NullJ
  deriving (Eq, Read)

instance Show ValueJ where
  show (ObjectJ map) = "{" ++ Map.foldrWithKey (\k o ac -> show k ++ ":" ++ show o ++ col ac ++ ac) "}" map
    where
      col "}" = ""
      col _ = ","
  show (ArrayJ ary) = "[" ++ Prelude.foldr (\o ac -> show o ++ col ac ++ ac) "]" ary
    where
      col "]" = ""
      col _ = ","
  show (StringJ str) = "\"" ++ escape str ++ "\""
    where
      escape ('"'  : rs)  = '\\' : '"'  : escape rs
      escape ('\\' : rs)  = '\\' : '\\' : escape rs
      escape ('/'  : rs)  = '\\' : '/'  : escape rs
      escape ('\b' : rs)  = '\\' : 'b'  : escape rs
      escape ('\n' : rs)  = '\\' : 'n'  : escape rs
      escape ('\r' : rs)  = '\\' : 'r'  : escape rs
      escape ('\t' : rs)  = '\\' : 't'  : escape rs
      escape (c : rs) = let
        op = if Char.ord c `elem` [0x0020 .. 0x10ffff] then (\_ -> [c]) else ((++ "u") . printf "%04x")
        in op c ++ escape rs
      escape [] = ""
  show (NumberJ num) = show num
  show (BoolJ True) = "true"
  show (BoolJ False) = "false"
  show NullJ = "null"