Parsing JSON string into record in Haskell

Question

I\'m struggling to understand this (I\'m still a bit new to Haskell) but I\'m finding the documentation for the Text.JSON package to be a little confusing. Basicall

Answer 1

You need to write a showJSON and readJSON method, for your type, that builds your Haskell values out of the JSON format. The JSON package will take care of parsing the raw string into a JSValue for you.

Your tweet will be a JSObject containing a map of strings, most likely.

• Use show to look at the JSObject, to see how the fields are laid out.
• You can lookup each field using get_field on the JSObject.
• You can use fromJSString to get a regular Haskell strings from a JSString.

Broadly, you'll need something like,

{-# LANGUAGE RecordWildCards #-}

import Text.JSON
import Text.JSON.Types

instance JSON Tweet where

    readJSON (JSObject o) = return $ Tweet { .. }
            where from_user         = grab o "from_user"
                  to_user_id        = grab o "to_user_id"
                  profile_image_url = grab o "proile_image_url"
                  created_at        = grab o "created_at"
                  id_str            = grab o "id_str"
                  source            = grab o "source"
                  to_user_id_str    = grab o "to_user_id_str"
                  from_user_id_str  = grab o "from_user_id_str"
                  from_user_id      = grab o "from_user_id"
                  text              = grab o "text"
                  metadata          = grab o "metadata"


grab o s = case get_field o s of
                Nothing            -> error "Invalid field " ++ show s
                Just (JSString s') -> fromJSString s'

Note, I'm using the rather cool wild cards language extension.

Without an example of the JSON encoding, there's not much more I can advise.

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Related

You can find example instances for the JSON encoding via instances

• in the source, for simple types. Or in other packages that depend on json.
• An instance for AUR messages is here, as a (low level) example.