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Statements must return unit #23

blast-hardcheese opened this issue May 19, 2019 · 2 comments


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commented May 19, 2019

The following causes some noise during migration to this plugin:

import cats.implicits._
case class Foo(a: Long)
for {
  Foo(a) <- Either.right[String, Foo](Foo(1))
  b <- Either.right(Foo(2))
  Foo(c) = b
} yield a + c


scala> for { Foo(a) <- Either.right[String, Foo](Foo(1)); b <- Either.right(Foo(2)); Foo(c) = b } yield a + c
<console>:17: warning: [wartremover:NonUnitStatements] Statements must return Unit
       for { Foo(a) <- Either.right[String, Foo](Foo(1)); b <- Either.right(Foo(2)); Foo(c) = b } yield a + c
res1: scala.util.Either[String,Long] = Right(3)

Walking through the tree, I can't even see where this error is coming from. By spacing out the above code, I can narrow it down to:

<console>:20: warning: [wartremover:NonUnitStatements] Statements must return Unit
         Foo(c) = b

Any pointers or insight would be appreciated. This is already excellent, just looking to learn.

The simplified tree from the SBT console is:

object $iw extends scala.AnyRef {
  def <init>() = {
  val res1 = Either.right[String, Foo](Foo(1)).withFilter((check$ifrefutable$1) =>
    check$ifrefutable$1: @scala.unchecked match {
      case Foo((a @ _)) => true
      case _ => false
  .flatMap((x$4) =>
    x$4: @scala.unchecked match {
      case Foo((a @ _)) =>
          .map((b) => {
            <synthetic> <artifact> private[this] val x$2 = b: @scala.unchecked match {
              case (x$1 @ Foo((c @ _))) => scala.Tuple2(x$1, c)
            val x$1 = x$2._1;
            val c = x$2._2;
            scala.Tuple2(b, x$1)
          .map((x$3) =>
            x$3: @scala.unchecked match {
              case scala.Tuple2((b @ _), Foo((c @ _))) => a.$plus(c)

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commented May 21, 2019

That tree looks like one between parser and bm4 first phase, so it won't tell you anything. What you want to check is tree right after typer or bm4-typer

It's probably caused by tuple removal feature, which, to support -Ywarn-unused:vars and side-effects, has to wrap stuff in locally (see comment in the source, and there is no stdlib function that gives you an ability to discard value.

So, your best bet is probably disabling that feature by adding -P:bm4:no-tupling:n to scalac options. Or, if that doesn't help, try to figure out which features are causing the issues.

I'll take a look if I can add a SuppressWarning for WartRemover just to a block later on, but don't hold your breath on that for now.


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commented May 21, 2019

Thanks for the explanation, I'll give it a shot!

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