No supertype allowed in handler return

[marzipankaiser]

The following program gives the annotated error:

interface Done {
  def done(): Nothing
}

def main() = {
  try {
    do done()
  } with Done {
    def done() = ()   // Error: Expected Nothing but got Unit.
  }
}

Is this the behaviour we want?

Note that this also disallows, e.g., a hole instead of do done().

[b-studios]

What would you expect, otherwise? The answer type of the handler is Nothing unless you upcast it there.

We could of course compute the join of the different handlers and the answer type.

[phischu]

I'll bump this and add the following example:

effect emit[A](value: A): Unit
effect fork(): Bool
effect fail(): Nothing

def select[A] { stream: () => Unit / emit[A] }: A / {fork, fail} =
  try {
    stream()
    do fail()
  } with emit[A] { value =>
    if (do fork()) {
      return value
    } else {
      resume(())
    }
  }

We should compute the join of the handlers and the answer type, yes.

[phischu]

As a workaround, the following does typecheck:

def select[A] { stream: () => Unit / emit[A] }: A / {fork, fail} = {
  def body(): A / emit[A] = {
    stream()
    do fail()
  }
  try {
    body()
  } with emit[A] { value =>
    if (do fork()) {
      return value
    } else {
      resume(())
    }
  }
}

After inlining body we are back to the original which does not.