sub foo {
  my $self = shift;
  if (@_) {
    if (defined wantarray) {
      croak "can't expect a return value when setting foo";
    }
    # store foo = shift;
  }
  else {
    unless (defined wantarray) {
      croak "must expect a return value when fetching foo";
    }
    # return foo member here
  }
}