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Today's Topics:
1. Parse Json tweets using Aeson (Tushar Jarhad)
2. Re: Parse Json tweets using Aeson (Michael Alan Dorman)
3. Re: Parse Json tweets using Aeson (Michael Alan Dorman)
----------------------------------------------------------------------
Message: 1
Date: Fri, 19 Sep 2014 03:53:50 +0530
From: Tushar Jarhad <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] Parse Json tweets using Aeson
Message-ID:
<CAH5gX+49=N2B39th0=o1_ytffvbyc+czhrwces_hgiult2m...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric #-}
import Data.Aeson
import GHC.Generics
import qualified Data.ByteString.Lazy as B
data Tweet = Tweet {
id_str :: String,
text :: String
}deriving Generic
instance FromJSON Tweet
main = do
input <- B.readFile "1.json"
let mm = decode input :: Maybe Tweet
case mm of
Nothing -> print "error parsing JSON"
Just m -> (putStrLn.greet) m
greet m = (show.id_str) m
This program is fine if 1.json file has 1 tweet
if 1.json file has multiple tweets then it fails
so how we can parse multiple tweets
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Message: 2
Date: Fri, 19 Sep 2014 04:40:55 -0400
From: Michael Alan Dorman <[email protected]>
To: Tushar Jarhad <[email protected]>
Cc: Haskell Beginners <[email protected]>
Subject: Re: [Haskell-beginners] Parse Json tweets using Aeson
Message-ID: <[email protected]>
Content-Type: text/plain
Tushar Jarhad <[email protected]> writes:
> let mm = decode input :: Maybe Tweet
You have told the program you expect one tweet---anything else is, by
definition, an error. The program takes you at your word.
So if you want it to parse multiple tweets, tell it that you expect a
*list* of tweets.
> let mm = decode input :: Maybe [Tweet]
Or are you saying that your file simply has multiple tweets appended one
after another? If so, that file does not represent a valid JSON
structure.
I feel sure there's a way to run the parse incrementally---taking a
complete data structure at a time---but I don't know it offhand.
Mike.
------------------------------
Message: 3
Date: Fri, 19 Sep 2014 04:51:24 -0400
From: Michael Alan Dorman <[email protected]>
To: Tushar Jarhad <[email protected]>
Cc: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Parse Json tweets using Aeson
Message-ID: <[email protected]>
Content-Type: text/plain
Michael Alan Dorman <[email protected]> writes:
> I feel sure there's a way to run the parse incrementally---taking a
> complete data structure at a time---but I don't know it offhand.
Since I felt like I *should* know this offhand, I decided to figure it
out. It's very easy---Aeson provides an Attoparsec parser, so you can,
in fact, do parsing-with-leftovers with virtually zero effort:
import Data.Aeson
import Data.Attoparsec.ByteString
import Data.ByteString
var :: ByteString
var = "{\"Some\" : \"Text\"}Leftover"
main = print $ parse json var
This produces a value representing the simple Object, as well as
leftovers---so you could, presumably, do a fold to pull out as many JSON
items as can be parsed, even if the overall file isn't a valid JSON
structure in itself.
Mike.
------------------------------
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