The Scala Toolkit

How to serialize an object to JSON?

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    Using Scala CLI, you can require the entire toolkit in a single line:

    //> using toolkit latest

    Alternatively, you can require just a specific version of UPickle:

    //> using dep com.lihaoyi::upickle:3.1.0

    In your build.sbt file, you can add the dependency on the Toolkit:

    lazy val example = project.in(file("."))
  .settings(
    scalaVersion := "3.3.4",
    libraryDependencies += "org.scala-lang" %% "toolkit" % "0.1.7"
  )

    Alternatively, you can require just a specific version of UPickle:

    libraryDependencies += "com.lihaoyi" %% "upickle" % "3.1.0"

    In your build.sc file, you can add the dependency to the upickle library:

    object example extends ScalaModule {
  def scalaVersion = "3.3.4"
  def ivyDeps =
    Agg(
      ivy"org.scala-lang::toolkit:0.1.7"
    )
}

    Alternatively, you can require just a specific version of UPickle:

    ivy"com.lihaoyi::upickle:3.1.0"

    Serializing a Map to JSON

    uPickle can serialize your Scala objects to JSON, so that you can save them in files or send them over the network.

    By default it can serialize primitive types such as Int or String, as well as standard collections such as Map and List.

    val map: Map[String, Int] =
  Map("Toolkitty" -> 3, "Scaniel" -> 5)
val jsonString: String = upickle.default.write(map)
println(jsonString)
// prints: {"Toolkitty":3,"Scaniel":5}

    Serializing a custom object to JSON

    In Scala, you can use a case class to define your own data type. For example, to represent a pet owner with the name of its pets, you can

    case class PetOwner(name: String, pets: List[String])

    To be able to write a PetOwner to JSON we need to provide a ReadWriter instance for the case class PetOwner. Luckily, upickle is able to fully automate that:

    import upickle.default._

implicit val ownerRw: ReadWriter[PetOwner] = macroRW[PetOwner]

    Some explanations:

    • An implicit val is a value that can be automatically provided as an argument to a method or function call, without having to explicitly pass it.
    • macroRW is a method provided by uPickle that can generate a instances of ReadWriter for case classes, using the information about its fields.
    import upickle.default.*

case class PetOwner(name: String, pets: List[String]) derives ReadWriter

    The derives keyword is used to automatically generate given instances. Using the compiler’s knowledge of the fields in PetOwner, it generates a ReadWriter[PetOwner].

    This means that you can now write (and read) PetOwner objects to JSON with upickle.default.write(petOwner).

    Notice that you do not need to pass the instance of ReadWriter[PetOwner] explicitly to the write method. But it does, nevertheless, get it from the context, as “given” value. You may find more information about contextual abstractions in the Scala 3 Book.

    Putting everything together you should get:

    import upickle.default._

case class PetOwner(name: String, pets: List[String])
implicit val ownerRw: ReadWriter[PetOwner] = macroRW

val petOwner = PetOwner("Peter", List("Toolkitty", "Scaniel"))
val json: String = write(petOwner)
println(json)
// prints: {"name":"Peter","pets":["Toolkitty","Scaniel"]}"

    import upickle.default._

case class PetOwner(name: String, pets: List[String]) derives ReadWriter

val petOwner = PetOwner("Peter", List("Toolkitty", "Scaniel"))
val json: String = write(petOwner)
println(json)
// prints: {"name":"Peter","pets":["Toolkitty","Scaniel"]}"
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