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{- Streaming JSON output.
-
- Copyright 2011, 2016 Joey Hess <id@joeyh.name>
-
- License: BSD-2-clause
-}
{-# LANGUAGE GADTs #-}
module Utility.JSONStream (
JSONChunk(..),
start,
add,
end
) where
import Data.Aeson
import qualified Data.Text as T
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)]
AesonObject :: Object -> JSONChunk Object
encodeJSONChunk :: JSONChunk v -> B.ByteString
encodeJSONChunk (JSONChunk l) = encode $ object $ map mkPair l
where
mkPair (s, v) = (T.pack s, toJSON v)
encodeJSONChunk (AesonObject o) = encode o
{- Aeson does not support building up a larger JSON object piece by piece
- 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 -> B.ByteString
start a
| not (B.null b) && B.last b == endchar = B.init b
| otherwise = bad b
where
b = encodeJSONChunk a
add :: JSONChunk a -> B.ByteString
add a
| not (B.null b) && B.head b == startchar = B.cons addchar (B.drop 1 b)
| otherwise = bad b
where
b = start a
end :: B.ByteString
end = endchar `B.cons` sepchar `B.cons` B.empty
startchar :: Word8
startchar = fromIntegral (ord '{')
endchar :: Word8
endchar = fromIntegral (ord '}')
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
|
