Let's write a very minimal TODO application providing a Json web API.

import lol.http._ import lol.json._ import lol.html._  

We will use circe as Json library for this example. You should first have a look a the circe documentation to understand the basics.

import io.circe._ import io.circe.syntax._ import io.circe.generic.auto._ import io.circe.optics.JsonPath._ import cats.implicits._ import cats.effect.{ IO } import scala.util._ import scala.concurrent.ExecutionContext.Implicits.global  

object JsonWebService {  

First, let's define a Todo case class. We let the circe macro automatically generate the corresponding Json encoder/decoder for this type.

case class Todo(id: Int, text: String, done: Boolean)  

Also, we define an ordering for our todo values so we always list them is the right order.

implicit val mostRecentTodoFirst: Ordering[Todo] = Ordering.by(todo => -todo.id)  

And here is a simple helper that will be used to generate Error Json values.

def Error(msg: String): Json = Json.obj("error" -> Json.fromString(msg))  


This is our state. We use a concurrent Map for storing the data, and an atomic counter to generate the identifiers sequence.

val todos = collection.concurrent.TrieMap.empty[Int,Todo] val idCounter = new java.util.concurrent.atomic.AtomicInteger(0)  


Let's start by defining the Html part of our app. It is a PartialService because it does not respond to all the incoming requests.

lazy val App: PartialService = { case GET at url"/" => 

By using the tmpl interpolation, we generate a very minimal UI 🎷.

Ok(tmpl""" <h1>@if(todos.isEmpty) {Nothing} else {A ton of things} to do</h1> <ol> @todos.values.toSeq.sorted.map { case Todo(_, text, done) => <li style="text-decoration: @if(done) { "line-through" } else { "none" }">@text</li> } <ol> """) }  


Then we define the Json API part. It contains all the endpoint needed to create, list, update and delete todos.

lazy val Api: PartialService = {  

This matcher will only match GET for the /api/todos path. If a done parameter is present in the incoming queryString it will be bound to the doneFilter value.

case GET at url"/api/todos?done=$doneFilter" => Try(doneFilter.toBoolean).toOption match

If the done parameter value was missing or was not a boolean value we return everything.

case None => Ok(todos.values.toSeq.sortBy(_.id).asJson

Otherwise we extract the actual list of todos and we transform it into a Json array.

case Some(isDone) => Ok(todos.values.toSeq.filter(_.done == isDone).sorted.asJson) }  

For this one, we actually need to get a reference of the request value, because we want to consume it's json content.

case request @ POST at url"/api/todos" => 

We first try to read to content into a Json value (this is an asynchronous operation and the Future will fail if the Json can't be parsed).

Then we use Circe optics to extract the text field from the incoming Json.

request.readAs[Json].map(root.text.string.getOption).map(_.get).map { text => val nextId = idCounter.incrementAndGet val newTodo = Todo(nextId, text, false) todos += (nextId -> newTodo) Created(newTodo.asJson) }.recover { case _ => BadRequest(Error("Please specify the `text' field for this item.")) }  

Nothing special here, but look how we handle the 404 case.

case GET at url"/api/todos/$id" => IO { Try(id.toInt).toOption.flatMap(todos.get).map { todo => Ok(todo.asJson) }.getOrElse { NotFound(Error(s"No todo found for id: `$id'")) } }  

Again we need to keep a reference on the request in order to read the json body.

case request @ POST at url"/api/todos/$id" => Try(id.toInt).toOption.flatMap(todos.get).map { todo => request.readAs[Json].map { jsonBody => val updatedTodo = todo.copy( text = root.text.string.getOption(jsonBody).getOrElse(todo.text), done = root.done.boolean.getOption(jsonBody).getOrElse(todo.done) ) todos += (todo.id -> updatedTodo) Ok(updatedTodo.asJson) } }.getOrElse { NotFound(Error(s"No todo found for id: `$id'")) }  

Our last route handle deletion of the todos. Because deletion is idempotent we succeed even if the id was not found.

case DELETE at url"/api/todos/$id" => todos -= Try(id.toInt).toOption.getOrElse(-1) Ok }  

def main(args: Array[String]): Unit = { 

It is now time to actually bootstrap the application. We start an HTTP server, and we compose the App and Api functions we just defined.

Server.listen(8888)(App.orElse(Api).orElse { case _ => NotFound })  

Done! We can use a Json client to interact with the app.

println("Listening on http://localhost:8888...") } }