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

[Baa]

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 <aquagnu at gmail.com> 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
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