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1+{-# LANGUAGE OverloadedStrings #-}
2+
3+-- Simplest example of parsing and encoding JSON with Aeson.
4+
5+-- Above, we enable OverloadedStrings to allow a literal string (e.g. "name")
6+-- to be automatically converted to Data.Text.
7+-- This is useful when using Aeson's functions such as (.:) which expect Text.
8+-- Without it we'd need to use pack.
9+
10+import Data.Aeson
11+import qualified Data.Aeson.Types as T
12+
13+import Data.Attoparsec (parse, Result(..))
14+import Data.Text (Text)
15+import Control.Applicative ((<$>))
16+import Control.Monad (mzero)
17+import qualified Data.ByteString.Char8 as BS
18+-- Aeson's "encode" to JSON generates lazy bytestrings
19+import qualified Data.ByteString.Lazy.Char8 as BSL
20+
21+-- In main we'll parse a JSON message into a Msg and display that,
22+-- then we'll encode a different Msg as JSON, and display it.
23+main ::IO ()
24+main = do
25+ print $ parseMsgFromString exampleJSONMessage
26+ let reply = Msg "hello Aeson!"
27+ putStrLn $ "Encoded reply: " ++ BSL.unpack (encode reply)
28+
29+-- this is the type we'll be converting to and from JSON
30+data Msg = Msg Text deriving (Show)
31+
32+-- here's how we should parse JSON and construct a Msg
33+instance FromJSON Msg where
34+ parseJSON (Object v) = Msg <$> v .: "message"
35+ parseJSON _ = mzero
36+
37+-- here's how we should encode a Msg as JSON
38+instance ToJSON Msg where
39+ toJSON (Msg s) = object [ "message" .= s]
40+
41+-- Here's one way to actually run the parsers.
42+--
43+-- Note that we do two parses:
44+-- once into JSON then one more into our final type.
45+-- There are a number of choices when dealing with parse failures.
46+-- Here we've chosen to parse to Maybe Msg, and a Nothing will be returned
47+-- if parseJSON fails. (More informative options are available.)
48+--
49+-- This should take us (depending on success or failure)
50+-- from {"message": "hello world"} to Just (Msg "hello world")
51+-- or to Nothing
52+--
53+-- Note also that we have not checked here that the input has been completely
54+-- consumed, so:
55+-- {"message": "hello world"} foo BIG mistake
56+-- would yield the same successfully translated message!
57+-- We could look in "rest" for the remainder.
58+parseMsgFromString :: String -> Maybe Msg
59+parseMsgFromString s =
60+ let bs = BS.pack s
61+ in case parse json bs of
62+ (Done rest r) -> T.parseMaybe parseJSON r :: Maybe Msg
63+ _ -> Nothing
64+
65+-- Here's the example JSON message we're going to try to parse:
66+-- {"message": "hello world"}
67+-- It's a JSON object with a single pair, having key 'message', and a string value.
68+-- It could have more fields and structure, but that's all we're going to parse out of it.
69+exampleJSONMessage :: String
70+exampleJSONMessage = "{\"message\":\"hello world\"}"