bmjames / json4s-zipper

Pure functional transformations for the json4s and Play JSON ASTs

Version Matrix


This is an experimental zipper library for the json4s and Play JSON ASTs. It is designed to be easy to make compatible with the other JSON libraries, by implementing the JsonLike typeclass.

The goals of this library are twofold:

  • To implement purely functional modifications to immutable JSON structures;
  • To support writing functions that are reusable with various JSON libraries, including ones not yet in existence.

How to get it

To use with json4s (3.2.x):

libraryDependencies += "net.bmjames" %% "json-zipper-core" % "0.2"

libraryDependencies += "net.bmjames" %% "json-zipper-json4s" % "0.2"

To use with Play (2.3.x):

libraryDependencies += "net.bmjames" %% "json-zipper-core" % "0.2"

libraryDependencies += "net.bmjames" %% "json-zipper-play" % "0.2"


To start with, here is some JSON, parsed into the JValue AST from json4s. (You can start a REPL in which to follow these examples by running sbt "project test" test:console from the root of this source tree.)

import org.json4s.native.JsonMethods._

val json = parse("""{"soups":["goulash","gumbo","minestrone"]}""")

Cursor API (quite stable)

This is the core zipper data type upon which the other APIs are based. It's a little verbose, but you can use this API directly. Most operations result in an Option[Cursor[_]], as they may fail (e.g. if you use field, but the cursor is not currently on an object).

import net.bmjames.json.json4s._
import net.bmjames.json.syntax._
import org.json4s.JString

val cursor = json.cursor // A cursor focusing on the root of the JSON object

val updatedCursor = for {
  a <- cursor.field("soups")         // Go to field "soups"
  b <- a.prepend(JString("borscht")) // Prepend to the array
} yield b

for (c <- updatedCursor) println(compact(render(c.toJson)))
// {"soups":["borscht","goulash","gumbo","minestrone"]}

XPath-style syntax (quite experimental)

This library supports modification of a JSON structure using an xpath-like syntax.

// Append the string " is tasty!" to each string in the array within the field "soups"
val tastySoups = json.mod ("soups" \ *) { case JString(s) => JString(s + " is tasty!") }

// {"soups":["goulash is tasty!","gumbo is tasty!","minestrone is tasty!"]}

The XPath-like syntax is implemented using the CursorArrow API. See CursorArrowExamples for examples of using CursorArrow directly.

Lenses (quite stable)

Lenses enable bidirectional transformations on data structures; i.e. the ability to query and update a view of the structure, with modifications propagating back as changes to the original structure.

This library implements Scalaz partial lenses for any data type having a JsonLike typeclass instance. The get and putback operations are implemented using zippers.

The partiality of the lenses is a result of the potential absence of expected elements in the JSON structure. get and set operations return an Option, and mod operations which fail will return the original structure unmodified.

import net.bmjames.json.Lenses._

// A partial lens focusing on the string value of the 2nd element of field "soups"
val firstSoup = field("soups") >=> elem(1) >=> strVal

// The lens can be used simply to view the value at that location
// Some(gumbo)

// The lens can also be used to transform the value
val updatedJson = firstSoup.mod("shellfish " + _, json)

// {"soups":["goulash","shellfish gumbo","minestrone"]}

See LensExamples for more examples.