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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 qualified Text.JSON as JSON
import qualified Data.Aeson as Aeson
import qualified Data.ByteString.Lazy.UTF8 as B
{- Only JSON objects can be used as chunks in the stream, not
- other values.
-
- Both Aeson and Text.Json objects are supported. -}
data JSONChunk a where
JSONObject :: JSON.JSON a => [(String, a)] -> JSONChunk [(String, a)]
AesonObject :: Aeson.Object -> JSONChunk Aeson.Object
encodeJSONChunk :: JSONChunk a -> String
encodeJSONChunk (JSONObject l) = JSON.encodeStrict $ JSON.toJSObject l
encodeJSONChunk (AesonObject o) = B.toString (Aeson.encode o)
{- Text.JSON and Aeson do not support building up a larger JSON document
- piece by piece as a stream. To support streaming, a hack. The final "}"
- is left off the object, allowing it to be added to later. -}
start :: JSONChunk a -> String
start a
| last s == endchar = init s
| otherwise = bad s
where
s = 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
|
