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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.UTF8 as B
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 -> String
start a
| last s == endchar = init s
| otherwise = bad s
where
s = B.toString $ encodeJSONChunk a
add :: JSONChunk a -> String
add a
| head s == startchar = ',' : drop 1 s
| otherwise = bad s
where
s = start a
end :: String
end = [endchar, '\n']
startchar :: Char
startchar = '{'
endchar :: Char
endchar = '}'
bad :: String -> a
bad s = error $ "JSON encoder generated unexpected value: " ++ s
|
