stripped-aeson

A layer around the deriving-aeson package, with the ability to strip one or more fields from the JSON output, and recover them when decoding using some specified defaults.

Usage

Example

Because an example speaks more than a thousand words, let's start by having a look at this Main module:

{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE DerivingVia                #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE TypeApplications           #-}

module Main where

import Data.Aeson
import Data.Set (Set)
import Data.Text (Text)
import Deriving.Aeson.Stripped

newtype WrappedInt
    = WrappedInt Int
    deriving newtype (FromJSON, Num, Show, ToJSON)

data RecordTest = RecordTest
    { testBool     :: !Bool
    , testNumber   :: !Int
    , testNewtype  :: !WrappedInt
    , testString   :: !String
    , testIsString :: !Text
    , testList     :: ![Int]
    , testIsList   :: !(Set Int)
    , testMonoid   :: !Ordering
    , testValue    :: !Double
    }
    deriving (Generic, Show)
    deriving (FromJSON, ToJSON)
        via StrippedJSON
            '[ RField "testBool"     'False
             , RField "testNumber"   7
             , RField "testNewtype"  (Coerce 42 Int)
             , RField "testString"   "string"
             , RField "testIsString" (FromString "text")
             , RField "testList"     '[ 10, 11, 12 ]
             , RField "testIsList"   (FromList '[ 13, 14, 13 ])
             , RField "testMonoid"   Mempty
             ]
            '[ FieldLabelModifier CamelToSnake
             ]
            RecordTest

main :: IO ()
main = do
    let output = encode $ RecordTest True 1 2 "s" "t" [1..3] (fromList [4..6]) GT 3.14

    putStrLn "# Stripping fields:"
    print output
    putStrLn "# And recovering them later:"
    print $ decode @RecordTest output

Upon running the corresponding executable, you should get this output :

# Stripping fields:
"{\"test_value\":3.14}"
# And recovering them later:
Just (RecordTest {testBool = False, testNumber = 7, testNewtype = 42, testString = "string", testIsString = "text", testList = [10,11,12], testIsList = fromList [13,14], testMonoid = EQ, testValue = 3.14})

How does it work? The core type: StrippedJSON (and the others)

This package revolves entirely around the StrippedJSON newtype, which takes two type parameters:

• a list of instructions indicating which fields should be removed, and how they should be recovered:

  Instruction	Use for	Field identifier
  RField	record values	name
  CField	non-record single-constructor values	zero-based position

  Each instruction takes a field identifier and a default value used to recover the field when decoding.

  See the RecoverableValue instances for examples of what can be used as default values. Or don't hesitate to roll your own! (PRs welcome!)

• a list of AesonOptions indicating how to customize the JSON output.

  This works exactly the same as the only parameter to CustomJSON from the deriving-aeson package.

Haddock documentation

The Haddock documentation for the package is available here.