Re: [Haskell-beginners] How to parse JSON into existential type

Ohh, right, there is `Value`!! Thank you David for good idea!

Rather than having an existential, which is difficult to work with at the best of times, how about making RespItem into something concrete? You could make your own data type to represent strings and arrays and objects, etc, but you could also just use Value from aeson, since it already exists and already has a FromJSON instance.

data Resp = Resp { rHeadings :: [T.Text], rRow :: [Data.Aeson.Value] }

I couldn't get deriving Generic to work, but I've personally not had too much trouble writing aeson instances manually.

instance FromJSON Resp where parseJSON = withObject "Resp" $ \v -> Resp <$> v .: "headings" <*> v .: "rows"

There is also an aeson-lens package that can make parsing json easy, if you are into lenses.

On Tue, Aug 29, 2017 at 8:36 AM, Baa wrote:

...

Hello List!

In some of my JSONs I have part "list of any values", for example:

[1, "hi", "false", 2.71]

To parse it I create type like:

data RespItem = forall a. FromJSON a => RespItem { riItem :: [a] }

,------------------------------------------------------------------- | btw, I try `(FromJSON a, Show a, Generic a) => ...` also but don't | see any difference, so I'm not sure what is right constraints list. `-------------------------------------------------------------------

And GHC does not allow to derive automatically with "deriving" Show and Generic instances for existential types, as I see. So I write:

instance Generic RespItem instance Show RespItem

Also this type `RespItem` in part of another one:

data Resp = Resp { rHeadings :: [T.Text] , rRow :: [RespItem] } deriving (Generic, Show)

Interesting is that now auto-`deriving` works! OK, but I need to to instantiate `FromJSON` for RespItem because next step must be `FromJSON` for Resp. So I'm trying:

instance FromJSON RespItem where parseJSON ??? = ???

and here I get different errors, and absolutely don't understand how to parse it, I get "ambiguous type error..." and so on (which is right, sure).

=== Best regards, Paul _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

---

Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners