Finding a value in ByteString (which is actually JSON) - json

Question

A web service returns a response as ByteString
req <- parseUrl "https://api.example.com"
res <- withManager $ httpLbs $ configReq req
case (HashMap.lookup "result" $ responseBody res) of .... -- error - responseBody returns ByteString
where
configReq r = --......
To be more specific, responseBody returns data in ByteString, although it's actually valid JSON. I need to find a value in it. Obviously, it would be easier to find it if it was JSON and not ByteString.
If that's the case, how do I convert it to JSON?
UPDATE:
decode $ responseBody resp :: IO (Either String Aeson.Value)
error:
Couldn't match expected type `IO (Either String Value)'
with actual type `Maybe a0'

Answer 1

You'll find several resources for converting bytestring to JSON. The simplest use cases are on the hackage page itself, and the rest you can infer using type signatures of the entities involved.
https://hackage.haskell.org/package/aeson-0.7.0.6/docs/Data-Aeson.html
But here's a super quick intro to JSON with Aeson:
In most languages, you have things like this:
someString = '{ "name" : ["value1", 2] }'
aDict = json.loads(someString)
This is obviously great, because JSON has a nearly one to one mapping with a fundamental data-structure of the language. Containers in most dynamic languages can contain values of any type, and so moving from JSON to data structure is a single step.
However, that is not the case with Haskell. You can't put things of arbitrary types into a container like type (A list, or a dictionary).
So Aeson does a neat thing. It defines an intermediate Haskell type for you, that maps directly to JSON.
A fundamental unit in Aeson is a Value. The Value can contain many things. Like an integer, string, an array, or an object.
https://hackage.haskell.org/package/aeson-0.7.0.6/docs/Data-Aeson.html#t:Value
An aeson array is a Vector (like a list but better) of Values and an aeson object is a HashMap of Text to Values
The next interesting step is that you can define functions that will convert an Aeson value to your Haskell type. This completes the loop. ByteString to Value to a custom type.
So all you do is implement parseJSON and toJSON functions that convert aeson Values to your type and vice-versa. The bit that converts a bytestring into a valid aeson value is implemented by aeson. So the heavy lifting is all done.
Just important to note, that Aeson bytestring is a lazy bytestring, so you might need some strict to lazy helpers.
stringToLazy :: String -> ByteString
stringToLazy x = Data.Bytestring.Lazy.fromChunks [(Data.ByteString.Char8.pack x)]
lazyToString :: ByteString -> String
lazyToString x = Data.ByteString.Char8.unpack $ Data.ByteString.Char8.concat $ Data.ByteString.Lazy.toChunks
That should be enough to get started with Aeson.
--
Common decoding functions with Aeson:
decode :: ByteString -> Maybe YourType
eitherDecode :: ByteString -> Either String YourType.
In your case, you're looking for eitherDecode.