handle C-style escapes in Format

I was happily able to repurpose some code from Git.Filename to handle this.

I remember writing that code... a whole afternoon at a coffee shop, after
which I felt I'd struggled with Haskell and git, and sorta lost, in needing
to write this nasty peice of code. But was also pleased at the use of a
pair of functions and quickcheck that allowed me to get it 100% right.
So, turns out I not only got it right, but the code wasn't as special-purpose
as I'd feared. Yay!

Git/Filename.hs

@@ -8,10 +8,7 @@
 
 module Git.Filename where
 
-import qualified Codec.Binary.UTF8.String
-import Data.Char
-import Data.Word (Word8)
-import Text.Printf
+import Utility.Format (decode_c, encode_c)
 
 import Common
 
@@ -19,64 +16,12 @@ decode :: String -> FilePath
 decode [] = []
 decode f@(c:s)
 	-- encoded strings will be inside double quotes
-	| c == '"' && end s == ['"'] = unescape ("", beginning s)
+	| c == '"' && end s == ['"'] = decode_c $ beginning s
 	| otherwise = f
-	where
-		e = '\\'
-		unescape (b, []) = b
-		-- look for escapes starting with '\'
-		unescape (b, v) = b ++ fst pair ++ unescape (handle $ snd pair)
-			where
-				pair = span (/= e) v
-		isescape x = x == e
-		-- \NNN is an octal encoded character
-		handle (x:n1:n2:n3:rest)
-			| isescape x && alloctal = (fromoctal, rest)
-				where
-					alloctal = isOctDigit n1 &&
-						isOctDigit n2 &&
-						isOctDigit n3
-					fromoctal = [chr $ readoctal [n1, n2, n3]]
-					readoctal o = Prelude.read $ "0o" ++ o :: Int
-		-- \C is used for a few special characters
-		handle (x:nc:rest)
-			| isescape x = ([echar nc], rest)
-			where
-				echar 'a' = '\a'
-				echar 'b' = '\b'
-				echar 'f' = '\f'
-				echar 'n' = '\n'
-				echar 'r' = '\r'
-				echar 't' = '\t'
-				echar 'v' = '\v'
-				echar a = a
-		handle n = ("", n)
 
 {- Should not need to use this, except for testing decode. -}
 encode :: FilePath -> String
-encode s = foldl (++) "\"" (map echar s) ++ "\""
-	where
-		e c = '\\' : [c]
-		echar '\a' = e 'a'
-		echar '\b' = e 'b'
-		echar '\f' = e 'f'
-		echar '\n' = e 'n'
-		echar '\r' = e 'r'
-		echar '\t' = e 't'
-		echar '\v' = e 'v'
-		echar '\\' = e '\\'
-		echar '"'  = e '"'
-		echar x
-			| ord x < 0x20 = e_num x -- low ascii
-			| ord x >= 256 = e_utf x
-			| ord x > 0x7E = e_num x -- high ascii
-			| otherwise = [x]        -- printable ascii
-			where 
-				showoctal i = '\\' : printf "%03o" i
-				e_num c = showoctal $ ord c
-				-- unicode character is decomposed to
-				-- Word8s and each is shown in octal
-				e_utf c = showoctal =<< (Codec.Binary.UTF8.String.encode [c] :: [Word8])
+encode s = "\"" ++ encode_c s ++ "\""
 
 {- for quickcheck -}
 prop_idempotent_deencode :: String -> Bool