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This package provides genericEncodeJson and genericDecodeJson functions for any data types that have a Generic.Rep instance, which can be used to encode or decode Json values or to implement EncodeJson and DecodeJson instances for argonaut-codecs.


Install argonaut-generic with Spago:

spago install argonaut-generic

Quick start

We can use functions from the Data.Argonaut.Decode.Generic.Rep and Data.Argonaut.Encode.Generic.Rep modules to automatically write instances of DecodeJson and EncodeJson for our data types. The below example defines a recursive sum type, derives Generic, and then uses this library to write its decoding and encoding instances:

import Prelude

import Data.Argonaut.Decode.Class (class DecodeJson)
import Data.Argonaut.Decode.Generic.Rep (genericDecodeJson)
import Data.Argonaut.Encode.Class (class EncodeJson)
import Data.Argonaut.Encode.Generic.Rep (genericEncodeJson)
import Data.Either (Either(..))
import Data.Generic.Rep (class Generic)

data Example
  = Either (Either String Example)
  | Record { foo :: Int, bar :: String }
  | Nested { foo :: { nested :: Int }, bar :: String }
  | Product Int Int Example

-- We first need to derive `Generic` for our type
derive instance genericExample :: Generic Example _

-- Necessary to eta-expand because the generic data type is recursive.
instance encodeJsonExample :: EncodeJson Example where
  encodeJson a = genericEncodeJson a

instance decodeJson :: DecodeJson Example where
  decodeJson a = genericDecodeJson a

You may also be interested in other libraries in the Argonaut ecosystem:

  • purescript-argonaut-core defines the Json type, along with basic parsing, printing, and folding functions
  • purescript-argonaut-codecs provides codecs based on EncodeJson and DecodeJson type classes, along with instances for common data types and combinators for encoding and decoding Json values.
  • purescript-argonaut-traversals provides prisms, traversals, and a zipper for working with nested Json structures.


argonaut-generic documentation is stored in a few places:

  1. Module documentation is published on Pursuit.
  2. Written documentation is kept in the docs directory.
  3. Usage examples can be found in the test suite.

If you get stuck, there are several ways to get help:


You can contribute to argonaut-generic in several ways:

  1. If you encounter a problem or have a question, please open an issue. We'll do our best to work with you to resolve or answer it.

  2. If you would like to contribute code, tests, or documentation, please read the contributor guide. It's a short, helpful introduction to contributing to this library, including development instructions.

  3. If you have written a library, tutorial, guide, or other resource based on this package, please share it on the PureScript Discourse! Writing libraries and learning resources are a great way to help this library succeed.