Type mismatch with Binding.scala involving scala.xml.Elem

Question

I'm failing to compile a simple example of Binding.scala, and being a newbie, I have no intuition how to fix it. Maybe the README is slightly outdated? The example at https://github.com/ThoughtWorksInc/Binding.scala-sample is even older and causes deprecation warnings.

My code, which I basically stuck together from the README, and even simplified a bit:

import com.thoughtworks.binding.dom
import org.scalajs.dom.document
import scala.scalajs.js.annotation.JSExport

@JSExport
object SampleMain {

  @dom
  def table = {
    <table border="1" cellPadding="5">
      <thead>
        <tr>
          <th>Name</th>
          <th>E-mail</th>
        </tr>
      </thead>
      <tbody>
      </tbody>
    </table>
  }

  @JSExport
  def main(): Unit = {
    dom.render(document.body, table)
  }

}

fastOptJS causes a compile error:

SampleMain.scala:25:9: overloaded method value render with alternatives:
[error]   (parent: org.scalajs.dom.raw.Node,children: com.thoughtworks.binding.Binding[com.thoughtworks.binding.Binding.BindingSeq[org.scalajs.dom.raw.Node]],dummy: Unit)Unit <and>
[error]   (parent: org.scalajs.dom.raw.Node,children: com.thoughtworks.binding.Binding.BindingSeq[org.scalajs.dom.raw.Node])Unit <and>
[error]   (parent: org.scalajs.dom.raw.Node,child: com.thoughtworks.binding.Binding[org.scalajs.dom.raw.Node])Unit
[error]  cannot be applied to (org.scalajs.dom.raw.HTMLElement, scala.xml.Elem)
[error]     dom.render(document.body, table)
[error]         ^

I suspected a problem with type inference and tried this type annotation: def table: com.thoughtworks.binding.Binding[org.scalajs.dom.html.Table] but this caused another error:

SampleMain.scala:11:6: type mismatch;
[error]  found   : scala.xml.Elem
[error]  required: com.thoughtworks.binding.Binding[org.scalajs.dom.html.Table]
[error]     (which expands to)  com.thoughtworks.binding.Binding[org.scalajs.dom.raw.HTMLTableElement]
[error]     <table border="1" cellPadding="5">
[error]      ^

I'd appreciate an explanation what is going wrong here.

Solution: https://stackoverflow.com/a/55137909/1862339

stholzm · Accepted Answer · 2019-03-13T09:16:01.800

Turns out the problem was that the paradise compiler plugin was not picked up in my case. I'm building an SBT multi-project with Binding.scala in a sub-project only, and addCompilerPlugin does not propagate to sub-projects. To make it work, it needs to be added to the sub-project's settings like this:

lazy val client = (project in file("client"))
  .settings(
    libraryDependencies ++= Seq(
      "com.thoughtworks.binding" %%% "dom" % "11.6.0"
    ),
    addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)
  )
  .enablePlugins(ScalaJSPlugin)

Before I had addCompilerPlugin at the top-level of build.sbt, which did not work and caused the compile error.

score 2 · Answer 2 · answered Mar 12 '19 at 18:36

2

Check that you have added this to your build.sbt

addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

The main macro of Binding.scala (which translates Scala XML literals into special Binding types) does not work without this plugin and the compiler sees only the original types (scala.xml.Elem).

This is mentioned in the Step 1 of the README.

answered Mar 12 '19 at 18:36

lmars

302
1
6

Thanks for pushing me into the right direction! My build was a bit more complicated and I did (try to) configure the compiler plugin, but it was not correct (see my own answer). Your insight really helped me to understand the problem. – stholzm Mar 13 '19 at 09:09

score 2 · Answer 3 · answered Mar 13 '19 at 03:06

There should be an error message, which asks you to enable the paradise compiler plug-in. Unfortunately because you are using Scala 2.10, a bug in macro-compat prevent you to see the error message.

So the long answer is:

Upgrade Scala to 2.11 or 2.12, by setting something like scalaVersion in ThisBuild := "2.12.8" in your build.sbt.
You will see an error message that asks you to add the paradise plug-in.
Finally, add the following setting in your build.sbt according to the error message:

addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

Actually I was using Scala 2.12 but did not see the helpful error message. Maybe because I am using an SBT multi-project build? Anyways, the compiler plugin was the hint I needed, I fixed a misconfiguration in SBT (see my answer). Thanks! — stholzm, Mar 13 '19 at 09:14

Type mismatch with Binding.scala involving scala.xml.Elem

3 Answers3