XO0540: a Mutex.with write that an Err undoes

A Mutex.with closure’s changes to its var value are written back only when it returns successfully; an Err (a return Err(...), a ?, or a call that can fail as the closure’s result) leaves the protected value as it was. A sticky error, a done flag, or consumed input stored before the failure is lost with it. To keep the change and still report the failure, return the outcome as the closure’s success value (its type Result[T, E] with no !) and apply ? outside the with.

Example

fn fail(m: Mutex[St], e: E) -> () ! E {
  m.with(fn(var st) -> () ! E {
    st.err = Some(e)
    Err(e)
  })
}

Fix

fn fail(m: Mutex[St], e: E) -> () ! E {
  let r = m.with(fn(var st) -> Result[(), E] {
    st.err = Some(e)
    Err(e)
  })
  r?
}

Run xo explain XO0540 for this text in a terminal, or see section 10.6 of the specification.