Question

I am trying to parse JSON of the following form using aeson

{"field":{"name":"..."}}

or

{"tag":{"name":"..."}}

or

{"line":{"number":"..."}}

to construct the following data type

data Rule = Line Integer
          | Field L.ByteString
          | Tag L.ByteString

Unfortunately, I face two problems that I’ve not found solutions to, namely:

1. How do I parse nested JSON? Looking at the implementation of (.:), it uses lookup to extract a specific key’s value. I’m hesitant to do something like this as it seems to be relying too much on the specifics of how aeson implements things. Am I wrong in thinking this is an issue?

2. How do I use the correct data constructor based on which key is present in the JSON? All my efforts with <|> have led me nowhere.

I would post the code I’ve written thus far, but I haven’t even gotten to the point where I have anything worth posting.

Answer 1

How about the following?

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative
import           Data.Aeson
import           Data.Aeson.Types
import qualified Data.ByteString      as B
import qualified Data.ByteString.Lazy as L
import qualified Data.Map             as M

data Rule = Line Integer
          | Field L.ByteString
          | Tag L.ByteString
          deriving Show

instance FromJSON Rule where
  parseJSON j = do
    o <- parseJSON j -- takes care of JSON type check
    case M.toList (o :: Object) of
      [("field", Object o')] -> Field <$> o' .: "name"
      [("tag",   Object o')] -> Tag   <$> o' .: "name"
      [("line",  Object o')] -> Line  <$> o' .: "number"
      _                      -> fail "Rule: unexpected format"