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scalacontinuations

Scala: compilation error when declaring continuation of type Any => Nothing

This code gives compilation error:

import scala.util.continuations._

object CTest {
    def loop: Nothing = reset {
        shift {c: (Unit => Nothing) => c()}
        loop
    }

   def main(argv: Array[String]) {loop}
}

Error message:

    error: type mismatch;
 found   : ((Unit) => Nothing) => (Unit) => Nothing
 required: ((Unit) => B) => (Unit) => Nothing

But this code works as expected:

import scala.util.continuations._

object CTest {
    def loop: Nothing = reset {
        shift {c: (Unit => Any) => c.asInstanceOf[Unit => Nothing]()}
        loop
    }

   def main(argv: Array[String]) {loop}
}

The question is: why Scala compiler hates me continuations of type Any => Nothing?

Solution

  • It compiles if I specify the type arguments:

    shift[Unit, Nothing, Nothing] {c: (Unit => Nothing) => c()}

    It looks to me like the compiler should infer that B is Nothing, but it doesn't.