Question

Following on from a previous post, I’ve found I’m totally stuck. I’m trying to parse a JSON structure into my own type, and not only am I stuck on how to parse the Array, I’m not even sure if I’m using the Aeson library as intended. Any help would be greatly appreciated.

The code:

data Exif = Exif [(T.Text, ExifValue)] deriving (Show)
data ExifValue = 
    ExifText T.Text | 
    ExifInt Integer | 
    ExifDouble Double | 
    ExifBool Bool | 
    ExifArray [ExifValue] 
    deriving (Show)

instance FromJSON ExifValue where
    parseJSON (Number (I n)) = return $ ExifInt n
    parseJSON (Number (D n)) = return $ ExifDouble n
    parseJSON (String s)     = return $ ExifText s
    parseJSON (Bool b)       = return $ ExifBool b
    -- parseJSON (Array a)      = ?????

instance FromJSON Exif where
    parseJSON (Object o) = do
        x <- sequence $ map f (M.assocs o)
        return $ Exif x
        where 
        f (t, x) = do
            y <- parseJSON x 
            return ((t, y) :: (T.Text, ExifValue))

parseExifFile = fmap parseExifData . B.readFile

parseExifData :: B.ByteString -> Data.Attoparsec.Result (Data.Aeson.Result [Exif])
parseExifData content = parse (fmap fromJSON json) content

The test file:

[{
  "SourceFile": "test.jpg",
  "ExifTool:ExifToolVersion": 8.61,
  "File:FileName": "test.jpg",
  "File:FileSize": 2174179,
  "File:FileModifyDate": "2011:07:27 16:53:49-07:00",
  "File:FilePermissions": 644,
  "File:FileType": "JPEG",
  "File:MIMEType": "image/jpeg",
  "File:ExifByteOrder": "MM",
  "File:CurrentIPTCDigest": "32d6a77098a73aa816f2570c9472735a",
  "File:ImageWidth": 2592,
  "File:ImageHeight": 1936,
  "File:EncodingProcess": 0,
  "File:BitsPerSample": 8,
  "File:ColorComponents": 3,
  "File:YCbCrSubSampling": "2 2",
  "XMP:Subject": ["alpha","beta","gamma"]
}]

Answer 1

You have to follow the type of parseJSON a little bit down a rabbit trail, but once you recognize what (Array a) represents, it should be straightforward.

parseJSON has type Value -> Parser a, so (Array a) has type Value. One of the variants in the Value type is Array Array, so the a in (Array a) must be of the type Array, which is defined as Vector Value. The Values inside that Vector are what you want to call parseJSON on to return your list, so check out what you can do with a Vector.

The easiest approach would probably to convert a to a list with Vector.toList, and then use mapM to parse the Values.

Alternately, you could avoid the Vector to list conversion by changing your ExifArray variant to hold Vector ExifValue, and then using Vector.mapM.