decoding URI-encoded strings in Haskell

URIDecoding.hs

import Data.Bits ((.|.),(.&.),shiftL)
import Data.Char (chr, isHexDigit, digitToInt)
import Data.Text (Text, pack, unpack)

{-
sources : 

uri-encode (https://hackage.haskell.org/package/uri-encode-1.5.0.7/docs/src/Network.URI.Encode.html#decodeText)

network-uri (https://hackage.haskell.org/package/network-uri-2.6.4.2/docs/src/Network.URI.html#unEscapeString)
-}


-- | URI decode a 'Text', unicode aware.
decodeText :: Text -> Text
decodeText = pack . unEscapeString . unpack

-- |Turns all instances of escaped characters in the string back
--  into literal characters.
--
unEscapeString :: String -> String
unEscapeString [] = ""
unEscapeString s@(c:cs) = case unEscapeByte s of
    Just (byte, rest) -> unEscapeUtf8 byte rest
    Nothing -> c : unEscapeString cs

unEscapeByte :: String -> Maybe (Int, String)
unEscapeByte ('%':x1:x2:s) | isHexDigit x1 && isHexDigit x2 =
    Just (digitToInt x1 * 16 + digitToInt x2, s)
unEscapeByte _ = Nothing

-- Adapted from http://hackage.haskell.org/package/utf8-string
-- by Eric Mertens, BSD3
unEscapeUtf8 :: Int -> String -> String
unEscapeUtf8 c rest
    | c < 0x80 = chr c : unEscapeString rest
    | c < 0xc0 = replacement_character : unEscapeString rest
    | c < 0xe0 = multi1
    | c < 0xf0 = multi_byte 2 0xf 0x800
    | c < 0xf8 = multi_byte 3 0x7 0x10000
    | c < 0xfc = multi_byte 4 0x3 0x200000
    | c < 0xfe = multi_byte 5 0x1 0x4000000
    | otherwise    = replacement_character : unEscapeString rest
    where
    replacement_character = '\xfffd'
    multi1 = case unEscapeByte rest of
      Just (c1, ds) | c1 .&. 0xc0 == 0x80 ->
        let d = ((fromEnum c .&. 0x1f) `shiftL` 6) .|.  fromEnum (c1 .&. 0x3f)
        in if d >= 0x000080 then toEnum d : unEscapeString ds
                            else replacement_character : unEscapeString ds
      _ -> replacement_character : unEscapeString rest

    multi_byte :: Int -> Int -> Int -> String
    multi_byte i mask overlong =
      aux i rest (unEscapeByte rest) (c .&. mask)
      where
        aux 0 rs _ acc
          | overlong <= acc && acc <= 0x10ffff &&
            (acc < 0xd800 || 0xdfff < acc)     &&
            (acc < 0xfffe || 0xffff < acc)      = chr acc : unEscapeString rs
          | otherwise = replacement_character : unEscapeString rs

        aux n _ (Just (r, rs)) acc
          | r .&. 0xc0 == 0x80 = aux (n-1) rs (unEscapeByte rs)
                               $! shiftL acc 6 .|. (r .&. 0x3f)

        aux _ rs _ _ = replacement_character : unEscapeString rs