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CI Release Pursuit Maintainer: garyb

Prisms, traversals, and zipper for the Json type. Part of the Argonaut collection of libraries for working with JSON in PureScript.


Install argonaut-traversals with Spago:

spago install argonaut-traversals

or install as part of the Argonaut bundle:

spago install argonaut

Quick start

You can use the prisms defined in Data.Argonaut.Prisms with functions from the profunctor-lenses library to work with nested Json structures. For example:

// FFI file
exports.sampleJson = { "a": { "b": [ 10, 11, 12 ] } }
module Main where

import Prelude

import Effect (Effect)
import Data.Argonaut.Core (Json)
import Data.Argonaut.Prisms (_Array, _Number, _Object)
import Data.Maybe (Maybe(..))
import Data.Lens (preview)
import Data.Lens.Index (ix)
import Effect.Console (log)

foreign import sampleJson :: Json

main :: Effect Unit
main =
  -- Walk through an object at the key 'a', then an object at the key 'b', then
  -- get the first index of an array as a number.
  case preview (_Object <<< ix "a" <<< _Object <<< ix "b" <<< _Array <<< ix 0 <<< _Number) sampleJson of
    Nothing -> log "nothin' there"
    Just v -> log $ "This should be 10.0 " <> show v

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-codec-argonaut supports an alternative approach for codecs, which are based on profunctors instead of type classes.
  • purescript-argonaut-generic supports generic encoding and decoding for any type with a Generic instance.


argonaut-traversals 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-traversals 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.