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Can traits have secondary constructors in Scala 3?

Time：12-31

I copied the following code form the Auxiliary Class Constructors article, pasted into Scastie, changed class to trait and set the Scala version to 3.1.0:

val DefaultCrustSize = 12
val DefaultCrustType = "THIN"

// the primary constructor
trait Pizza (var crustSize: Int, var crustType: String) {

    // one-arg auxiliary constructor
    def this(crustSize: Int) = {
        this(crustSize, DefaultCrustType)
    }

    // one-arg auxiliary constructor
    def this(crustType: String) = {
        this(DefaultCrustSize, crustType)
    }

    // zero-arg auxiliary constructor
    def this() = {
        this(DefaultCrustSize, DefaultCrustType)
    }

    override def toString = s"A $crustSize inch pizza with a $crustType crust"

}

Here's the result. It gives the following error:

org.scalameta.invariants.InvariantFailedException: invariant failed:
when verifying scala.meta.classifiers.`package`.XtensionClassifiable[scala.meta.Template](templ)(scala.meta.Tree.classifiable[scala.meta.Template]).is[Template.Quasi](Template.this.Quasi.ClassifierClass[scala.meta.Template]).||(templ.stats.forall(((x$9: scala.meta.Stat) => scala.meta.classifiers.`package`.XtensionClassifiable[scala.meta.Stat](x$9)(scala.meta.Tree.classifiable[scala.meta.Stat]).is[Ctor](scala.meta.Ctor.ClassifierClass[scala.meta.Stat]).`unary_!`)))
found that scala.meta.classifiers.`package`.XtensionClassifiable[scala.meta.Template](templ)(scala.meta.Tree.classifiable[scala.meta.Template]).is[Template.Quasi](Template.this.Quasi.ClassifierClass[scala.meta.Template]) is false
and also templ.stats.forall(((x$9: scala.meta.Stat) => scala.meta.classifiers.`package`.XtensionClassifiable[scala.meta.Stat](x$9)(scala.meta.Tree.classifiable[scala.meta.Stat]).is[Ctor](scala.meta.Ctor.ClassifierClass[scala.meta.Stat]).`unary_!`)) is false
where Template = scala.meta.Template$@7759c8f1
where templ = {

    // one-arg auxiliary constructor
    def this(crustSize: Int) = {
        this(crustSize, DefaultCrustType)
    }

    // one-arg auxiliary constructor
    def this(crustType: String) = {
        this(DefaultCrustSize, crustType)
    }

    // zero-arg auxiliary constructor
    def this() = {
        this(DefaultCrustSize, DefaultCrustType)
    }

    override def toString = s"A $crustSize inch pizza with a $crustType crust"

}
    at org.scalameta.invariants.InvariantFailedException$.raise(Exceptions.scala:19)
    at scala.meta.Defn$Trait$.internal$247(Trees.scala:432)
    at scala.meta.Defn$Trait$.apply(Trees.scala:425)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$traitDef$1(ScalametaParser.scala:4592)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.traitDef(ScalametaParser.scala:4570)
    at scala.meta.internal.parsers.ScalametaParser.tmplDef(ScalametaParser.scala:4548)
    at scala.meta.internal.parsers.ScalametaParser.defOrDclOrSecondaryCtor(ScalametaParser.scala:4201)
    at scala.meta.internal.parsers.ScalametaParser.nonLocalDefOrDcl(ScalametaParser.scala:4172)
    at scala.meta.internal.parsers.ScalametaParser$$anonfun$templateStat$1$1.applyOrElse(ScalametaParser.scala:5120)
    at scala.meta.internal.parsers.ScalametaParser$$anonfun$templateStat$1$1.applyOrElse(ScalametaParser.scala:5114)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:35)
    at scala.meta.internal.parsers.ScalametaParser.statSeq(ScalametaParser.scala:5057)
    at scala.meta.internal.parsers.ScalametaParser.templateStats(ScalametaParser.scala:5130)
    at scala.meta.internal.parsers.ScalametaParser.templateStatSeq(ScalametaParser.scala:5110)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$templateBody$1(ScalametaParser.scala:4945)
    at scala.meta.internal.parsers.ScalametaParser.inBraces(ScalametaParser.scala:800)
    at scala.meta.internal.parsers.ScalametaParser.templateBody(ScalametaParser.scala:4945)
    at scala.meta.internal.parsers.ScalametaParser.templateBodyOpt(ScalametaParser.scala:4953)
    at scala.meta.internal.parsers.ScalametaParser.template(ScalametaParser.scala:4887)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$template$1(ScalametaParser.scala:4906)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.autoPos(ScalametaParser.scala:922)
    at scala.meta.internal.parsers.ScalametaParser.template(ScalametaParser.scala:4891)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$templateOpt$1(ScalametaParser.scala:4931)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.autoPos(ScalametaParser.scala:922)
    at scala.meta.internal.parsers.ScalametaParser.templateOpt(ScalametaParser.scala:4923)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$objectDef$1(ScalametaParser.scala:4682)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.objectDef(ScalametaParser.scala:4674)
    at scala.meta.internal.parsers.ScalametaParser.tmplDef(ScalametaParser.scala:4558)
    at scala.meta.internal.parsers.ScalametaParser.topLevelTmplDef(ScalametaParser.scala:4540)
    at scala.meta.internal.parsers.ScalametaParser$$anonfun$topStat$1.applyOrElse(ScalametaParser.scala:5079)
    at scala.meta.internal.parsers.ScalametaParser$$anonfun$topStat$1.applyOrElse(ScalametaParser.scala:5067)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:35)
    at scala.meta.internal.parsers.ScalametaParser.statSeq(ScalametaParser.scala:5057)
    at scala.meta.internal.parsers.ScalametaParser.topStatSeq(ScalametaParser.scala:5066)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$batchSource$1(ScalametaParser.scala:5297)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.autoPos(ScalametaParser.scala:922)
    at scala.meta.internal.parsers.ScalametaParser.batchSource(ScalametaParser.scala:5256)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$source$1(ScalametaParser.scala:5239)
    at scala.meta.internal.parsers.ScalametaParser.atPos(ScalametaParser.scala:888)
    at scala.meta.internal.parsers.ScalametaParser.autoPos(ScalametaParser.scala:922)
    at scala.meta.internal.parsers.ScalametaParser.source(ScalametaParser.scala:5238)
    at scala.meta.internal.parsers.ScalametaParser.entrypointSource(ScalametaParser.scala:5244)
    at scala.meta.internal.parsers.ScalametaParser.$anonfun$parseSource$2(ScalametaParser.scala:143)
    at scala.meta.internal.parsers.ScalametaParser.parseRule(ScalametaParser.scala:53)
    at scala.meta.internal.parsers.ScalametaParser.parseSource(ScalametaParser.scala:143)
    at scala.meta.parsers.Parse$.$anonfun$parseSource$1(Parse.scala:29)
    at scala.meta.internal.parsers.ScalametaParser$$anon$264.apply(ScalametaParser.scala:5308)
    at scala.meta.parsers.Api$XtensionParseDialectInput.parse(Api.scala:25)
    at scala.meta.parsers.Api$XtensionParseInputLike.parse(Api.scala:14)
    at com.olegych.scastie.instrumentation.Instrument$.apply(Instrument.scala:199)
    at com.olegych.scastie.instrumentation.InstrumentedInputs$.apply(InstrumentedInputs.scala:23)
    at com.olegych.scastie.sbt.SbtProcess$$anonfun$4.applyOrElse(SbtProcess.scala:191)
    at com.olegych.scastie.sbt.SbtProcess$$anonfun$4.applyOrElse(SbtProcess.scala:177)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:35)
    at akka.actor.FSM.processEvent(FSM.scala:707)
    at akka.actor.FSM.processEvent$(FSM.scala:704)
    at com.olegych.scastie.sbt.SbtProcess.processEvent(SbtProcess.scala:64)
    at akka.actor.FSM.akka$actor$FSM$$processMsg(FSM.scala:701)
    at akka.actor.FSM$$anonfun$receive$1.applyOrElse(FSM.scala:695)
    at akka.actor.Actor.aroundReceive(Actor.scala:539)
    at akka.actor.Actor.aroundReceive$(Actor.scala:537)
    at com.olegych.scastie.sbt.SbtProcess.aroundReceive(SbtProcess.scala:64)
    at akka.actor.ActorCell.receiveMessage(ActorCell.scala:614)
    at akka.actor.ActorCell.invoke(ActorCell.scala:583)
    at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:268)
    at akka.dispatch.Mailbox.run(Mailbox.scala:229)
    at akka.dispatch.Mailbox.exec(Mailbox.scala:241)
    at akka.dispatch.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
    at akka.dispatch.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
    at akka.dispatch.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
    at akka.dispatch.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

Why? And why is it after line 1, which seems perfectly fine and gets accepted when I remove the rest? Does "invariant failed" mean that it's a bug in the compiler? (Many other compilers in such cases add to the message an encouragement to report it.)

Main questions:

  • Are secondary (or auxiliary – IIUC the two terms I've seen used apparently interchangeably mean the same) constructors allowed in traits?
  • If so, how to call them?
  • What are they useful for (or would be, or are in another language that actually has them)?

CodePudding user response：

The documentation on trait parameters only mentions trait parameters, not trait constructors:

Scala 3 allows traits to have parameters, just like classes have parameters.

It also links to the original SIP document for reference:

For more information, see Scala SIP 25.

In the SIP-25 – trait parameters, it says [bold emphasis mine]:

In the ClassDef of traits, we still do not allow secondary constructors.

However, this restriction, is not reflected in the Syntax Summary, which does not actually distinguish between classes and traits syntactically. So, the restriction is purely one of documentation, not syntax specification.

So, to answer your question:

Why?

Because your code is syntactically valid but semantically invalid, and ScalaMeta seems to not expect this specific kind of semantic invalidity.

And why is it after line 1, which seems perfectly fine and gets accepted when I remove the rest? Does "invariant failed" mean that it's a bug in the compiler? (Many other compilers in such cases add to the message an encouragement to report it.)

It's clearly not a bug in the compiler since the exception is not thrown in the compiler, it is thrown in ScalaMeta.

Main questions:

  • Are secondary (or auxiliary – IIUC the two terms I've seen used apparently interchangeably mean the same) constructors allowed in traits?

No. SIP-25 clearly disallows them.

Page link：https//www.codepudding.com/Softwaredesign/246081.html

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