How to parse complex JSON in haskell

Question

I am creating a JSON parser in Haskell. I have followed this link https://hackage.haskell.org/package/aeson-0.8.0.0/docs/Data-Aeson.html and tried to parse the JSON string and got successful as there was a single object.But now i have to parse the data below But the problem is how to parse this large data as it contains objects and nested objects thus i cannot understand how to parse it, Please help me in this regard. Any sort of help will be highly appreciated!!

"apiVersion": "2.0",
"data": {
"updated": "2010-01-07T19:58:42.949Z",
"totalItems": 800,
"startIndex": 1,
"itemsPerPage": 1,
"items": [
{
"id": "hYB0mn5zh2c",
"uploaded": "2007-06-05T22:07:03.000Z",
"updated": "2010-01-07T13:26:50.000Z",
"uploader": "GoogleDeveloperDay",
"category": "News",
"title": "Google Developers Day US - Maps API Introduction",
"description": "Google Maps API Introduction ...",
"tags": [
"GDD07",
"GDD07US",
"Maps"
],
"thumbnail": {
"default": "http://i.ytimg.com/vi/hYB0mn5zh2c/default.jpg",
"hqDefault": "http://i.ytimg.com/vi/hYB0mn5zh2c/hqdefault.jpg"
},
"player": {
"default": "http://www.youtube.com/watch?vu003dhYB0mn5zh2c"
},
"content": {
"1": "rtsp://v5.cache3.c.youtube.com/CiILENy.../0/0/0/video.3gp",
"5": "http://www.youtube.com/v/hYB0mn5zh2c?f...",
"6": "rtsp://v1.cache1.c.youtube.com/CiILENy.../0/0/0/video.3gp"
},
"duration": 2840,
"aspectRatio": "widescreen",
"rating": 4.63,
"ratingCount": 68,
"viewCount": 220101,
"favoriteCount": 201,
"commentCount": 22,
"status": {
"value": "restricted",
"reason": "limitedSyndication"
},
"accessControl": {
"syndicate": true,
"commentVote": true,
"rate": true,
"list": true,
"comment": true,
"embed": true,
"videoRespond": "moderated"
}
}
]
}

to give a complete solution is quite some work - so maybe you can add what you tried so far or simplify this into a simple example with one nested object - I'm sure you'll get a good answer soon you can expand on your own — Random Dev
– Random Dev, Commented Nov 2, 2014 at 19:14
If you can just tell how to declare this array "data": { "updated": "2010-01-07T19:58:42.949Z", "totalItems": 800, "startIndex": 1, "itemsPerPage": 1, that will be helpfull!! — k102041 Hamza Rais
– k102041 Hamza Rais, Commented Nov 2, 2014 at 20:04

jamshidh · Accepted Answer · 2014-11-02 19:59:50Z

1

First of all, I want to note that your data isn't valid JSON.... It is missing enclosing curly brackets.... Add "{" at the beginning and "}" at the end to fix this.

Once you have that, using Data.Aeson to parse is very easy to do, just use decode. There is a hitch, though. decode has an overloaded output type, and you will have to specify which output you are interested in. To just get an answer, you can use Maybe Value.

decode dataByteString::Maybe Value

Note that the data needs to be in a ByteString (hence my naming).

The Data.Aeson package can be used to do much more, including parse to a specific non-generic type, but you haven't asked for that, so I won't get into it.

A small test program to parse at the command line would be:

import Data.Aeson
import qualified Data.ByteString.Lazy.Char8 as BLC

decodeToMaybeValue::BLC.ByteString->Maybe Value
decodeToMaybeValue = decode

main = do
  interact (show . decodeToMaybeValue . BLC.pack)

The only reason you need to redefine decode to decodeToMaybeValue is to specify its output type.

answered Nov 2, 2014 at 19:59

jamshidh

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5 Comments

k102041 Hamza Rais Over a year ago

i have added the relevant libraries but i have now got stuck in the parsing code as i tried this instance FromJSON Detail where parseJSON (Object v) = Detail <$> v .: "id" <*> v .: "type1" <*> v .: "name" <*> v .: "ppu" --{-# LANGUAGE XOverloadedStrings #-} and it worked but for this any clue for parsing an array??

jamshidh Over a year ago

This is a modification of the original question, you should probably spin that out as a new question, and make the use case as small as possible (ie- shrink the input data to something as small as possible that still has the problem).... I guarantee that a lot of people here will help if you do this.

k102041 Hamza Rais Over a year ago

what is the way to parse this data as i can compile it on the compiler directly but cannot compile it by loading it.

jamshidh Over a year ago

@k102041HamzaRais- I don't understand the question.... The code I gave you parses the data, the full version will compile, then you have to pipe the file into it (ie- in Linux/bash, type cat data.json | ./parseData, where the created executable is parseData.

Mittenchops Over a year ago

"The Data.Aeson package can be used to do much more, including parse to a specific non-generic type, but you haven't asked for that, so I won't get into it" are there questions that address this clearly?

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