Cherry picking in JSON objects.

Simple single record picking out of nested JSON objects

jsop, JSON record cherry picker

JSOP is good for picking out a product type value from nested json objects

The jread memoize the keys path structure so jread f g should be curried to repeat on multiple values. The Value will be scanned only one time, despite the paths are always expressed from the root. Order is restored by a final lookup.

Example

Preamble

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeApplications #-}

import Data.Aeson
import Data.Aeson.Lens
import Data.String.Interpolate
import qualified Data.Text as T
import Generics.SOP
import Generics.SOP.TH
import JSOP.Parse
import Protolude hiding (All, optional, (:*:))
import Data.Maybe (fromJust)

Given we have a SOP encoding of the record (tuples are good).

data ABC = ABC Text Int Int deriving (Show, Eq)

deriveGeneric ''ABC

Then we need a product of pickers with the same shape as our product type.

In this case I choose to encode paths joining json keys with /

cherryPickABC :: NP (Parser Text) '[Text, Int, Int]
cherryPickABC =
  required "object 1 / a string" _String
    :* required "object 2 / a number" _Integral
    :* optional "object 4 / a number" 42 _Integral
    :* Nil

Given the next json structure

jsonWithABC :: Value
jsonWithABC = fromJust . decode $ [i| 
  {
    "object 1": 
      { "a string": "ciao"
      , "ignore me" : 34
      }
  , "object 2": 
      { "a number": 2
      , "object 3": {}
      }
  , "object 4": {
      "a plumber" :43
      } 
  }
  |]

We can cherry pick the scattered ABC with

abc :: ABC
Right abc = jread (T.splitOn " / ") cherryPickABC jsonWithAB

We can also rewrite an ABC value in the json

Writing optionals has an ad-hoc defined behaviour, in general writing is still to be thought in case of missing keys

jsonABC :: Value 
jsonABC = jwrite (T.splitOn " / ") cherryPickABC jsonWithABC (ABC "mamma" 44 103)