JsZipper : Play2 Json advanced (& monadic) manipulations

JsZipper is a new tool allowing much more complex & powerful manipulations of Json structures for Play2/Json API:

JsZipper is inspired by the Zipper concept introduced by Gerard Huet in 1997.

The Zipper allows to update immutable traversable structures in an efficient way. Json is an immutable AST so it fits well. FYI, the Zipper behaves like a loupe that walks through each node of the AST (left/right/up/down) while keeping aware of the nodes on its left, its right and its upper. The interesting idea behind the loupe is that when it targets a node, it can modify and even delete the focused node. The analogy to the pants zipper is quite good too because when it goes down the tree, it behaves as if it was opening the tree to be able to drive the loupe through all nodes and when it goes up, it closes back the tree... I won't tell more here, it would be too long.

JsZipper is a specific interpretation of Zipper concept for Play/Json API based on :

  • Scala Streams to go through / update / construct Json AST in a lazy way
  • Monadic aspects to provide funnier ways of manipulating the Json AST (plz see below)

Please note, JsZipper is not an end in itself but a tool useful to provide new API to manipulate Json.

Use it in your SBT project

libraryDependencies ++= Seq(
  "io.github.hagay3" %% "play-json-zipper" % "2.0.1"


Let's go to samples.

We'll use following Json Object.

scala> import play.api.libs.json._
scala> import play.api.libs.json.monad.syntax._
scala> import play.api.libs.json.extensions._
scala> val js = Json.obj(
  "key1" -> Json.obj(
    "key11" -> "TO_FIND",
    "key12" -> 123L,
    "key13" -> JsNull
  "key2" -> 123,
  "key3" -> true,
  "key4" -> Json.arr("TO_FIND", 345.6, "test", Json.obj("key411" -> Json.obj("key4111" -> "TO_FIND")))
js: play.api.libs.json.JsObject = {"key1":{"key11":"TO_FIND","key12":123,"key13":null},"key2":123,"key3":true,"key4":["TO_FIND",345.6,"test",{"key411":{"key4111":"TO_FIND"}}]}

Basic manipulations

Setting multiple paths/values

scala> js.set(
  (__ \ "key4")(2) -> JsNumber(765.23),
  (__ \ "key1" \ "key12") -> JsString("toto")
res1: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND","key12":"toto","key13":null},"key2":123,"key3":true,"key4":["TO_FIND",345.6,765.23,{"key411":{"key4111":"TO_FIND"}}]}

Updating individual paths

scala> val json = Json.obj("key1" -> "toto")
json: play.api.libs.json.JsObject = {"key1":"toto"}

scala> val json = Json.obj("key1" -> "toto", "key2" -> Seq(3, 2, 1))
json: play.api.libs.json.JsObject = {"key1":"toto","key2":[3,2,1]}

scala> json.update(__ \ "key1", { case JsString(s) => JsString(s.toUpperCase); case other => other })
res1: play.api.libs.json.JsValue = {"key1":"TOTO","key2":[3,2,1]}

scala> json.updateAs[Seq[Int]](__ \ "key2", _.sorted)
res2: play.api.libs.json.JsValue = {"key1":"toto","key2":[1,2,3]}

Deleting multiple paths/values

scala> js.delete(
  (__ \ "key4")(2),
  (__ \ "key1" \ "key12"),
  (__ \ "key1" \ "key13")
res2: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND"},"key2":123,"key3":true,"key4":["TO_FIND",345.6,{"key411":{"key4111":"TO_FIND"}}]}

Finding paths/values according to a filter

scala> js.findAll( (_,v) => v == JsString("TO_FIND") ).toList
res5: List[(play.api.libs.json.JsPath, play.api.libs.json.JsValue)] = List(

Updating values according to a filter based on value

scala> js.updateAll( (_:JsValue) == JsString("TO_FIND") ){ js =>
  val JsString(str) = js
  JsString(str + "2")
res6: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND2","key12":123,"key13":null},"key2":123,"key3":true,"key4":["TO_FIND2",345.6,"test",{"key411":{"key4111":"TO_FIND2"}}]}

Updating values according to a filter based on path+value

scala> js.updateAll{ (path, js) =>
  JsPathExtension.hasKey(path) == Some("key4111")
}{ (path, js) =>
  val JsString(str) = js
  JsString(str + path.path.last)
res1: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND","key12":123,"key13":null},"key2":123,"key3":true,"key4":["TO_FIND",345.6,"test",{"key411":{"key4111":"TO_FIND/key4111"}}]}

Creating an object from scratch

scala> val build = JsExtensions.buildJsObject( 
  __ \ "key1" \ "key11" -> JsString("toto"),
  __ \ "key1" \ "key12" -> JsNumber(123L),
  (__ \ "key2")(0)      -> JsBoolean(true),
  __ \ "key3"           -> Json.arr(1, 2, 3)
build: play.api.libs.json.JsValue = {"key1":{"key11":"toto","key12":123},"key3":[1,2,3],"key2":[true]}

# Let's be funnier with Monads now

Let's use Future as our Monad because it's... coooool to do things in the future ;)

Imagine you call several services returning Future[JsValue] and you want to build/update a JsObject from it. Until now, if you wanted to do that with Play2/Json, it was quite tricky and required some code.

Here is what you can do now.

Updating multiple FUTURE values at given paths

scala> val maybeJs = js.setM[Future](
  (__ \ "key4")(2)        -> future{ JsNumber(765.23) },
  (__ \ "key1" \ "key12") -> future{ JsString("toto") }
maybeJs: scala.concurrent.Future[play.api.libs.json.JsValue] = scala.concurrent.impl.Promise$DefaultPromise@6beb722d

scala> Await.result(maybeJs, Duration("2 seconds"))
res4: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND","key12":"toto","key13":null},"key2":123,"key3":true,"key4":["TO_FIND",345.6,765.23,{"key411":{"key4111":"TO_FIND"}}]}

Update multiple FUTURE values according to a filter

scala> val maybeJs = js.updateAllM[Future]( (_:JsValue) == JsString("TO_FIND") ){ js =>
  future {
    val JsString(str) = js
    JsString(str + "2")
maybeJs: scala.concurrent.Future[play.api.libs.json.JsValue] = scala.concurrent.impl.Promise$DefaultPromise@35a4bb1a

scala> Await.result(maybeJs, Duration("2 seconds"))
res6: play.api.libs.json.JsValue = {"key1":{"key11":"TO_FIND2","key12":123,"key13":null},"key2":123,"key3":true,"key4":["TO_FIND2",345.6,"test",{"key411":{"key4111":"TO_FIND2"}}]}

Creating a FUTURE JsArray from scratch

scala> val maybeArr = JsExtensions.buildJsArrayM[Future](
  future { JsNumber(123.45) },
  future { JsString("toto") }
maybeArr: scala.concurrent.Future[play.api.libs.json.JsValue] = scala.concurrent.impl.Promise$DefaultPromise@220d48e4

scala> Await.result(maybeArr, Duration("2 seconds"))
res0: play.api.libs.json.JsValue = [123.45,"toto"]

Ok, much more can be done... Have fun!