avoid using Strings for JSON output; keep it ByteString throughout

Utility/JSONStream.hs

@@ -16,7 +16,9 @@ module Utility.JSONStream (
 
 import Data.Aeson
 import qualified Data.Text as T
-import qualified Data.ByteString.Lazy.UTF8 as B
+import qualified Data.ByteString.Lazy as B
+import Data.Char
+import Data.Word
 
 data JSONChunk v where
 	JSONChunk :: ToJSON v => [(String, v)] -> JSONChunk [(String, v)]
@@ -32,29 +34,35 @@ encodeJSONChunk (AesonObject o) = encode o
  - with streaming output. To support streaming, a hack:
  - The final "}" is left off the JSON, allowing more chunks to be added
  - to later. -}
-start :: JSONChunk a -> String
+start :: JSONChunk a -> B.ByteString
 start a
-	| last s == endchar = init s
-	| otherwise = bad s
+	| not (B.null b) && B.last b == endchar = B.init b
+	| otherwise = bad b
   where
-	s = B.toString $ encodeJSONChunk a
+	b = encodeJSONChunk a
 
-add :: JSONChunk a -> String
+add :: JSONChunk a -> B.ByteString
 add a
-	| head s == startchar = ',' : drop 1 s
-	| otherwise = bad s
+	| not (B.null b) && B.head b == startchar = B.cons addchar (B.drop 1 b)
+	| otherwise = bad b
   where
-	s = start a
+	b = start a
 
-end :: String
-end = [endchar, '\n']
+end :: B.ByteString
+end = endchar `B.cons` sepchar `B.cons` B.empty
 
-startchar :: Char
-startchar = '{'
+startchar :: Word8
+startchar = fromIntegral (ord '{')
 
-endchar :: Char
-endchar = '}'
+endchar :: Word8
+endchar = fromIntegral (ord '}')
 
-bad :: String -> a
-bad s = error $ "JSON encoder generated unexpected value: " ++ s
+addchar :: Word8
+addchar = fromIntegral (ord ',')
+
+sepchar :: Word8
+sepchar = fromIntegral (ord '\n')
+
+bad :: B.ByteString -> a
+bad b = error $ "JSON encoder generated unexpected value: " ++ show b