[Haskell-beginners] help understanding JSON

[lakshminaras2002 at gmail.com]

>Mostly this boils down to a complete lack of useful docs
True, I faced problems with respect to functions in System.Posix.IO because
of lack of proper docs.

On Tue, May 10, 2011 at 2:11 PM, Sean Perry <shaleh at speakeasy.net> wrote:

> Thanks Adrien. In that chapter they actually explicitly define all JS
> numbers to be Double and later in the Parsec chapter their parser reads the
> Strings into Floats directly. I was looking for a way to use the library
> provided parsers rather than crafting my own. Sure it is not much code but
> why rewrite what others have already tested?
>
> Mostly this boils down to a complete lack of useful docs. A laundry list of
> functions is not always  helpful. At least we could mimic the Python docs
> and add examples.
>
> On May 10, 2011, at 1:05, Adrien Haxaire <adrien at adrienhaxaire.org> wrote:
>
> > Hello,
> >
> > I do not have the right answer for your question, I am just starting to
> learn Haskell.
> >
> > Still, you may want to take a look at the JSON library example described
> in real world Haskell, this can bring you some more clues:
> >
> >
> http://book.realworldhaskell.org/read/writing-a-library-working-with-json-data.html
> >
> > Regards,
> > Adrien
> >
> >
> > On Tue, 10 May 2011 00:51:30 -0700, Sean Perry wrote:
> >> I have some JSON which looks like this:
> >>
> >> {"count":2, "result":[{"LastTrade":"31.24", "Symbol":"FOO"},
> >>                                  {"LastTrade":"345.12",
> "Symbol":"BAR"}]}
> >> (named testQuoteResult below)
> >>
> >> When I try to parse this into types defined below it failed because
> >> the Double is encoded as a String. If I change the type of LastTrade
> >> in my code to String everything is ok but I am left read'ing the
> >> String.
> >>
> >> I have defined the following using Language.JsonGrammar:
> >>
> >> import Data.Iso
> >> import Language.JsonGrammar
> >> import Prelude hiding (id, (.), head, either)
> >> import Control.Category ((.))
> >>
> >> import qualified Data.Aeson as JS
> >> import qualified Data.ByteString.Char8 as BS
> >> import qualified Data.Attoparsec as P
> >>
> >> data MyQuote = MyQuote { price  :: Double,
> >>                                                 symbol :: String }
> >>  deriving(Show)
> >>
> >> myQuote = $(deriveIsos ''MyQuote)
> >>
> >> instance Json MyQuote where
> >>  grammar = myQuote . object
> >>    (
> >>      prop "LastTrade"
> >>    . prop "Symbol"
> >>    )
> >>
> >> data MyQuoteResult = MyQuoteResult { count :: Int,
> >>
> >>  result :: [MyQuote] }
> >>  deriving(Show)
> >>
> >> myQuoteResult = $(deriveIsos ''MyQuoteResult)
> >>
> >> instance Json MyQuoteResult where
> >>  grammar = myQuoteResult . object
> >>    (  prop "count"
> >>    .  prop "result"
> >>    )
> >>
> >> tryJSON input =
> >>  case P.parse JS.json (BS.pack input) of
> >>    P.Done s v -> Right v
> >>    other      -> Left ("failed parse " ++ show other)
> >>
> >> Using it like this:
> >>> let Right qr = tryJSON testQuoteResult
> >>> fromJson qr :: Maybe MyQuoteResult
> >> Nothing
> >>
> >> Is there some way to convince the JSON text parser to turn "12.34"
> >> into (Double 12.34)??
> >>
> >> An example using pure aeson would be fine if required. I just decided
> >> to play with JsonGrammar since it was announced today and it sounded
> >> interesting.
> >>
> >>
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> >
> >
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-- 
Regards
Lakshmi Narasimhan T V
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