80 lines
1.8 KiB
Haskell
80 lines
1.8 KiB
Haskell
{- Streaming JSON output.
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-
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- Copyright 2011, 2016 Joey Hess <id@joeyh.name>
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-
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- License: BSD-2-clause
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-}
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{-# LANGUAGE GADTs, OverloadedStrings #-}
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module Utility.JSONStream (
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JSONChunk(..),
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start,
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add,
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addNestedObject,
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end
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) where
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import Data.Aeson
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import qualified Data.Text as T
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import qualified Data.ByteString.Lazy as B
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import qualified Data.ByteString.Lazy.UTF8 as BU8
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import Data.Char
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import Data.Word
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data JSONChunk v where
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AesonObject :: Object -> JSONChunk Object
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JSONChunk :: ToJSON v => [(String, v)] -> JSONChunk [(String, v)]
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encodeJSONChunk :: JSONChunk v -> B.ByteString
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encodeJSONChunk (AesonObject o) = encode o
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encodeJSONChunk (JSONChunk l) = encode $ object $ map mkPair l
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where
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mkPair (s, v) = (T.pack s, toJSON v)
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{- Aeson does not support building up a larger JSON object piece by piece
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- with streaming output. To support streaming, a hack:
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- The final "}" is left off the JSON, allowing more chunks to be added
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- to later. -}
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start :: JSONChunk a -> B.ByteString
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start a
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| not (B.null b) && B.last b == endchar = B.init b
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| otherwise = bad b
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where
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b = encodeJSONChunk a
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add :: JSONChunk a -> B.ByteString
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add a
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| not (B.null b) && B.head b == startchar =
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B.cons addchar (B.drop 1 b)
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| otherwise = bad b
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where
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b = start a
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addNestedObject :: String -> B.ByteString -> B.ByteString
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addNestedObject s b = B.concat
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[ ",\""
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, BU8.fromString s
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, "\":"
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, b
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, "}"
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]
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end :: B.ByteString
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end = endchar `B.cons` sepchar `B.cons` B.empty
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startchar :: Word8
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startchar = fromIntegral (ord '{')
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endchar :: Word8
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endchar = fromIntegral (ord '}')
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addchar :: Word8
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addchar = fromIntegral (ord ',')
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sepchar :: Word8
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sepchar = fromIntegral (ord '\n')
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bad :: B.ByteString -> a
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bad b = error $ "JSON encoder generated unexpected value: " ++ show b
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