Lens is cool

input.json

[
  {
    "name": "Lol inc",
    "departments": [
      {
        "name": "hr",
        "employees": [
          {
            "name": "Alice",
            "position": "manager",
            "salary": 4800
          },
          {
            "name": "Adam",
            "position": "worker",
            "salary": 3200
          },
          {
            "name": "Brenda",
            "position": "worker",
            "salary": 3000
          }
        ]
      },
      {
        "name": "factory",
        "employees": [
          {
            "name": "Bob",
            "position": "manager",
            "salary": 3500
          },
          {
            "name": "Kara",
            "position": "worker",
            "salary": 2620
          },
          {
            "name": "Erik",
            "position": "worker",
            "salary": 2500
          }
        ]
      }
    ]
  }
]

lens_aeson.hs

#!/usr/bin/env stack
{- stack
   script
   --install-ghc
   --resolver lts-12.18
   --package lens
   --package aeson
   --package text
   --package lens-aeson
   --package bytestring
-}

{-# language OverloadedStrings #-}

import Control.Lens (over, (^?), filtered)
import Data.Aeson.Lens (values, key, _String, _Number)
import Data.Text (Text)

import qualified Data.ByteString.Lazy
import qualified Data.ByteString.Lazy.Char8

main :: IO ()
main = do
  input <- Data.ByteString.Lazy.getContents
  -- give all workers a 5% raise:
  Data.ByteString.Lazy.putStr
    $ over (values.key "departments"
                  .values
                  .key "employees"
                  .values
                  .filtered (\employee ->
                                (employee ^? key "position"._String)
                                ==
                                Just "worker")
                  .key "salary"
                  ._Number)
           (* 1.05)
           input
  putStrLn ""

result.json

[
  {
    "name": "Lol inc",
    "departments": [
      {
        "name": "hr",
        "employees": [
          {
            "salary": 4800,
            "name": "Alice",
            "position": "manager"
          },
          {
            "salary": 3360,
            "name": "Adam",
            "position": "worker"
          },
          {
            "salary": 3150,
            "name": "Brenda",
            "position": "worker"
          }
        ]
      },
      {
        "name": "factory",
        "employees": [
          {
            "salary": 3500,
            "name": "Bob",
            "position": "manager"
          },
          {
            "salary": 2751,
            "name": "Kara",
            "position": "worker"
          },
          {
            "salary": 2625,
            "name": "Erik",
            "position": "worker"
          }
        ]
      }
    ]
  }
]

run.sh

cat input.json | ./lens_aeson.hs | jq . > result.json