Parsing a JSON string in Haskell

Question

I'm working on simple Haskell programme that fetches a JSON string from a server, parses it, and does something with the data. The specifics are not really pertinent for the moment, the trouble I'm having is with parsing the JSON that is returned.

I get the JSON string back from the server as an IO String type and can't seem to figure out how to parse that to a JSON object.

Any help would be much appreciated :)

Here is my code thus far.

import Data.Aeson
import Network.HTTP

main = do
    src <- openURL "http://www.reddit.com/user/chrissalij/about.json"
    -- Json parsing code goes here

openURL url = getResponseBody =<< simpleHTTP (getRequest url)

Note: I'm using Data.Aeson in the example as that is what seems to be recommended, however I'd be more than willing to use another library.

Also any and all of this code can be changed. If getting the

It would help for you to be more specific about what is holding you back. Is it getting the string out of the IO monad? Or is it the mechanics of parsing? (If it's the latter, check out RWH -- there are whole chapters devoted to parsing JSON.) FWIW, it'll be worth your while long-term to learn the ByteString library. Haskell Strings are very slow. — rtperson
– rtperson, Commented Aug 5, 2011 at 18:45
@rtperson. Strings are not automatically slow for parsing. As Strings are lists of Char they support destructuring at the head naturally and efficiently - this is exactly what parsing needs. Of course, lists of Char are a poor representation for general text regarding memory use. — stephen tetley
– stephen tetley, Commented Aug 5, 2011 at 21:06
@rtperson It's essentially both. I'm new to Haskell so while I've done JSON parsing in a few other languages, the specifics of Haskell are getting to me. — Chris Salij
– Chris Salij, Commented Aug 9, 2011 at 8:53

hammar · Accepted Answer · 2011-08-05 18:36:04Z

9

Data.Aeson is designed to be used with Attoparsec, so it only gives you a Parser that you must then use with Attoparsec. Also, Attoparsec prefers to work on ByteString, so you have to alter the way the request is made slightly to get a ByteString result instead of a String.

This seems to work:

import Data.Aeson
import Data.Attoparsec
import Data.ByteString
import Data.Maybe
import Network.HTTP
import Network.URI

main = do
    src <- openURL "http://www.reddit.com/user/chrissalij/about.json"
    print $ parse json src

openURL :: String -> IO ByteString
openURL url = getResponseBody =<< simpleHTTP (mkRequest GET (fromJust $ parseURI url))

Here I've just parsed the JSON as a plain Value, but you'll probably want to create your own data type and write a FromJSON instance for it to handle the conversion neatly.

answered Aug 5, 2011 at 18:36

hammar

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2 Comments

Chris Salij Over a year ago

This works like a charm. Thank you. I didn't see anywhere that I had to attoparsec :)

Chris Salij Over a year ago

Also I was trying to get a ByteString out, but was struggling with that too :P

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