The withXXX "helpers" make everything kind of awkward, but here goes.
The Aeson Parser type is misnamed, and this causes confusion.
The idea with Aeson Parser objects is that they represent a monadic parse result. (This is different from the Parser objects you find in Parsec, etc., which represent actual monadic parsers.) So, you should think of a Parser a as isomorphic to an Either ParseError a -- a monadic result with the possibility of failure.
These parse results are usually combined applicatively. So if you have a parser like:
data Xyz = Xyz { x :: String, y :: String }
instance FromJSON Xyz where
parseJSON = withObject "Xyz" $ \v ->
Xyz <$> v .: "x" <*> v .: "y"
the parse results v .: "x" and v .: "y" have type Parser String which is really like Either ParseError a, and the last line of that instance is the usual method of combining successful and unsuccessful results in an applicative manner, along the lines of:
Xyz <$> Right "value_x" <*> Left "while parsing Xyz: key y was missing"
Now, the function parseJSON has type Value -> Parser a. This is what should properly be called a parser, but to avoid confusion, let's call it a "parse function". A parse function takes a JSON representation (a Value, or an Object or some other JSON thingy) and returns a parse result. The withXXX family of functions are used to adapt parse functions between JSON thingies. If you have a parse function that expects an Object, like:
\v -> Xyz <$> v .: "x" <*> v .: "y" :: Object -> Parser Xyz
and you want to adapt it to parseJSON :: Value -> Parser Xyz, you use withObject "str" :: (Object -> Parser Xyz) -> (Value -> Parser Xyz) to do it.
Getting back to your problem, if you'd like to write a core parser that looks like:
\v -> Abc <$> v .: "name" <*> extra .:? "this_string_A"
<*> extra .:? "this_string_B"
you want extra to be an Object, and you want to extract it monadically from the overall JSON object v :: Object, using appropriate withXXX helpers to adapt parse functions from one input JSON thingy type to another. So, let's write a monadic function (a parse function, in fact) to do that:
getExtra :: Object -> Parser Object
getExtra v = do
First, we monadically extract the optional "extra" component from v. We use the conditional form here, so mextra :: Maybe Value.
mextra <- v .:? "extra"
Second, let's monadically create our final Object out of "mextra". This will be the JSON Object whose keys are "this_string_A" and "this_string_B" with the array layer removed. Note the type of this case expression will be Parser Object, a parse result of type Object = HashMap key value. For the Just case, we have a Value that we expect to be an array, so let's use the withArray helper to ensure that. Note that the withArray "str" helper function takes our parse function of type \arr -> do ... :: Array -> Parser Object and adapts it to Value -> Parser Object so it can be applied to vv :: Value.
case mextra of
Just vv -> vv & withArray "Abc.extra" (\arr -> do
Now, arr is an Array = Vector Value. We hope it's an array of Objects. Let's pull the Values out as a list:
let vallst = toList arr
and then monadically traverse the list with the help of withObject to ensure they're all Objects as expected. Note the use of pure here, since we want to extract the Objects as-is without any additional processing:
objlst <- traverse (withObject "Abc.extra[..]" pure) vallst
Now, we have an objlst :: [Object]. They're a set of singleton hashmaps with disjoint keys, and the Object / hashmap we want is their union, so let's return that. The parenthesis here ends the withArray expression that's being applied to vv:
return $ HashMap.unions objlst)
For the Nothing case ("extra" not found), we merely return an empty hashmap:
Nothing -> return HashMap.empty
The full function looks like this:
getExtra :: Object -> Parser Object
getExtra v = do
mextra <- v .:? "extra"
case mextra of
Just vv -> vv & withArray "Abc.extra" (\arr -> do
let vallst = toList arr
objlst <- traverse (withObject "Abc.extra[..]" pure) vallst
return $ HashMap.unions objlst)
Nothing -> return HashMap.empty
and you use it in your parser instance like so:
instance FromJSON Abc where
parseJSON =
withObject "Abc" $ \v -> do
extra <- getExtra v
Abc <$> v .: "name" <*> extra .:? "this_string_A" <*> extra .:? "this_string_B"
With a test case:
example :: BL.ByteString
example = "{\"name\": \"xyz1\", \"extra\": [{\"this_string_A\": \"Hello\"}, {\"this_string_B\": \"World\"}]}"
main = print (eitherDecode example :: Either String Abc)
it works like so:
λ> main
Right (Abc {name = "xyz1", a = Just "Hello", b = Just "World"})
The full code:
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
import Data.Aeson (eitherDecode, FromJSON, Object, parseJSON, withArray, withObject, (.:), (.:?))
import Data.Aeson.Types (Parser)
import GHC.Generics (Generic)
import qualified Data.ByteString.Lazy as BL (ByteString)
import qualified Data.HashMap.Strict as HashMap (empty, unions)
import Data.Function ((&))
import Data.Foldable (toList)
data Abc = Abc
{ name :: String
, a :: Maybe String
, b :: Maybe String
} deriving (Generic, Show)
instance FromJSON Abc where
parseJSON =
withObject "Abc" $ \v -> do
extra <- getExtra v
Abc <$> v .: "name" <*> extra .:? "this_string_A" <*> extra .:? "this_string_B"
getExtra :: Object -> Parser Object
getExtra v = do
mextra <- v .:? "extra"
case mextra of
Just vv -> vv & withArray "Abc.extra" (\arr -> do
let vallst = toList arr
objlst <- traverse (withObject "Abc.extra[..]" pure) vallst
return $ HashMap.unions objlst)
Nothing -> return HashMap.empty
example :: BL.ByteString
example = "{\"name\": \"xyz1\", \"extra\": [{\"this_string_A\": \"Hello\"}, {\"this_string_B\": \"World\"}]}"
main = print (eitherDecode example :: Either String Abc)
