Perl 6 also has dual vars

sub slick()
{
    if do-stuff
    {
      return IntStr.new: 1, "SomeValue";
    }
    else
    {
      return IntStr.new: 0, "Some error message";
    }
}

my $result = slick;
if +$result
{
    process( ~$result );
}
else
{
    log-error( ~$result );
}
[download]

I would not do this though, as you can pass around Failures just like any data type which but which act as undefined values.

sub slick()
{
    if do-stuff
    {
      return "SomeValue";
    }
    else
    {
      fail "Some error message";
    }
}

with slick() # is it defined
-> $result {
  process $result
}
else
-> $error {
  log-error $error.exception.message
}
[download]

Actually I would make it so that log-error accepts Failure objects, so that the .exception.message can be removed.

proto sub log-error ( $ ) {*}
multi sub log-error ( Str $message ) {
  say $message; # do the logging here
}
multi sub log-error ( Exception $exception ) {
  samewith $exception.message
}
multi sub log-error ( Failure $failure ) {
  samewith $failure.exception.message
}
[download]

You can also change out any method if you mixin a role.

# this has the same effect as `but False`
'Some error message' but role { method Bool ( --> False ) {} }
[download]

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