circe

Circe is a library which helps handle JSON format data. You can easily decode a JSON HTTP response to a case class. If you are working on API service, probably you will use it to encode your model with JSON format, then return to the API consumer.

Docode

Now, let's decode an http response, JSON data looks like:

{
  "userId": 1,
  "postId": 1,
  "title": "Start FP in Scala",
  "body": "This book means to be the first place when you try to learn Scala."
}

The Post model:

case class Post(userId: Int, postId: Int, title: String, body: String)

The decoder need to be implicit as required by circe. It needs to implements the Docoder trait with apply function implemented.

The input HCursor parameter for apply function is used to track the operation history of go to the field.

Decoder.Reuslt[A] is a type alias of Either[DecodingFailure, A]

Let's see decoder:

implicit object PostDecoder extends Decoder[Post] {
  override def apply(c: HCursor): Decoder.Result[Post] =
    for {
      userId <- c.get[Int]("userId")
      postId <- c.get[Int]("postId")
      title <- c.get[String]("title")
      body <- c.get[String]("body")
    } yield Post(userId, postId, title, body)
}

Use the above decoder to decode the input string:

io.circe.parser.decode(postString)

// result is:
// Right(Post(1, 1, "Start FP in Scala", "This book means to be the first place when you try to learn Scala."))

Encode

Imagine you already have the a Post instance, you want to present it through API in beautiful JSON format.

The encoder also needs to be implicit. Import the necessary circle syntax with import io.circe.syntax._.

implicit object PostEncoder extends Encoder[Post] {
    override def apply(a: Post): Json =
      Json.obj(
        "userId" -> a.userId.asJson,
        "postId" -> a.postId.asJson,
        "title" -> a.title.asJson,
        "content" -> a.body.asJson,
      )
  }

Check the ecoded result:

val post = Post(1, 1, "Start FP in Scala", "This book means to be the first place when you try to learn Scala.")

post.asJson.noSpaces

// result is:
// {"userId":1,"postId":1,"title":"Start FP in Scala","content":"This book means to be the first place when you try to learn Scala."}

Easy and cool, right?

Please write the decoder and encoder for the nested JSON data.