"Cannot find an implicit ExecutionContext" error in scala.js example app.

Question

Here is an example from the Hands-on Scala.js ebook:

package webpage

import org.scalajs.dom.ext.Ajax

import scala.scalajs.js
import scala.scalajs.js.annotation.JSExport
import scalatags.JsDom.all._
import org.scalajs.dom
import dom.html

@JSExport
object HelloWorld1 extends {
  @JSExport
  def main(target: html.Div) = {

    lazy val box = input(
      `type`:="text",
      placeholder:="Type here!"
    ).render

    lazy val output = div(
      height:="400px",
      overflowY:="scroll"
    ).render

    box.onkeyup = (e: dom.Event) => {
      output.innerHTML = "Loading..."
      fetchWeather(box.value)
    }

    target.appendChild(
      div(
        h1("Weather Search"),
        p(
          "Enter the name of a city to pull the ",
          "latest weather data from api.openweathermap.com!"
        ),
        p(box),
        hr, output, hr
      ).render
    )

    def fetchWeather(query: String) = {
      val searchUrl =
        "http://api.openweathermap.org/data/" +
          "2.5/find?type=like&mode=json&q=" +
          query

      for{
        xhr <- Ajax.get(searchUrl)
        if query == box.value
      } js.JSON.parse(xhr.responseText).list match{
        case jsonlist: js.Array[js.Dynamic] =>
          output.innerHTML = ""
          showResults(jsonlist, query)
        case _ =>
          output.innerHTML = "No Results"
      }
    }
    def showResults(jsonlist: js.Array[js.Dynamic], query: String) = {
      for (json <- jsonlist) {
        val name = json.name.toString
        val country = json.sys.country.toString
        val weather = json.weather.pop().main.toString

        def celsius(kelvins: js.Dynamic) = {
          kelvins.asInstanceOf[Double] - 273.15
        }.toInt

        val min = celsius(json.main.temp_min)
        val max = celsius(json.main.temp_max)
        val humid = json.main.humidity.toString
        val (first, last) = name.splitAt(query.length)
        output.appendChild(
          div(
            b(span(first, backgroundColor:="yellow"), last, ", ", country),
            ul(
              li(b("Weather "), weather),
              li(b("Temp "), min, " - ", max),
              li(b("Humidity "), humid, "%")
            )
          ).render
        )
      }
    }
  }
}

Got an error when compiling:

[info] Compiling 1 Scala source to /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/target/scala-2.11/classes...
[error] /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/src/main/scala/HelloWorld1.scala:51: Cannot find an implicit ExecutionContext. You might pass
[error] an (implicit ec: ExecutionContext) parameter to your method
[error] or import scala.concurrent.ExecutionContext.Implicits.global.
[error]         if query == box.value
[error]                  ^
[warn] /Users/kaiyin/personal_config_bin_files/workspace/workbench-example-app/src/main/scala/HelloWorld1.scala:53: non-variable type argument scala.scalajs.js.Dynamic in type pattern scala.scalajs.js.Array[scala.scalajs.js.Dynamic] is unchecked since it is eliminated by erasure
[warn]         case jsonlist: js.Array[js.Dynamic] =>
[warn]                           ^
[warn] one warning found
[error] one error found
[info] workbench: Checking example-fastopt.js
[info] workbench: Checking webpage-fastopt.js
[error] (compile:compile) Compilation failed
[error] Total time: 0 s, completed 7 sept. 2015 12:51:38
78. Waiting for source changes... (press enter to interrupt)

What went wrong here?

Answer 1

As the error suggests, add the following imports:

import scala.concurrent._
import ExecutionContext.Implicits.global