[Haskell-beginners] using record in aeson

12 Oct 2011

      Hi All:

I've been elaborating on aeson examples and wondered whether someone
could clarify the syntax for using a record in a pair. My goal is to
substitute a record for the list of pairs created through the data
constructor O [(T.Text, Value)] in MyPair below. Reason being to embed
the semantics of the json file into the record. To reproduce, just
uncomment the lines in the source below.

The json file structure is as follows:
{"outer":{"type":"literal","value":"rick"}}

Note my naive attempt in the commented lines returns the following
message from ghci. 'f0 b0' doesn't give me much to go on.

-- E1.hs:35:41:
--     Couldn't match expected type `MyRecord' with actual type `f0 b0'
--     In the expression: MyRecord <$> o'' .: "type" <*> o'' .: "value"
--     In the first argument of `R', namely
--       `(t, MyRecord <$> o'' .: "type" <*> o'' .: "value")'
--     In the expression: R (t, MyRecord <$> o'' .: "type" <*> o'' .:
"value")
-- Failed, modules loaded: none.

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Applicative
import Control.Monad (mzero)

import qualified Data.ByteString as B
import qualified Data.Map as M
import qualified Data.Text as T

import Data.Aeson
import qualified Data.Aeson.Types as J
import Data.Attoparsec

-- data MyRecord = MyRecord {s :: String, u :: String} deriving (Show)

data MyPair = O (T.Text, [(T.Text, Value)])
           -- | R (T.Text, MyRecord) 
              deriving (Show)

data ExifObject = ExifObject [MyPair]
                deriving Show

data Exif       = Exif [ExifObject]
                deriving Show

instance FromJSON ExifObject
  where
    parseJSON (Object o) = ExifObject <$> parseObject o
      where
        parseObject o' = return $ map toMyPair (M.assocs o')

        toMyPair (t, Object o'')= O (t, M.assocs o'')
--      toMyPair (t, Object o'')= R (t, MyRecord <$> o'' .: "type" <*>
o'' .: "value")
        toMyPair _              = error "unexpected"

    parseJSON _          = mzero

parseAll :: B.ByteString -> [ExifObject]
parseAll s = case (parse (fromJSON <$> json) s) of
  Done _ (Error err)  -> error err
  Done ss (Success e) -> e:(parseAll ss)
  _                   -> []

main :: IO ()
main = do s <- B.readFile "e1.json"
          let p = Exif $ parseAll s
          print p

--
Rick

[Haskell-beginners] using record in aeson

Rick Murphy