Function direction

src/plan/hector.rs:140–166

(prefix: &P, extender_variables: (Var, Var))

}

fn direction<P>(prefix: &P, extender_variables: (Var, Var)) -> Result<Direction, &'static str>
where
    P: AsBinding + std::fmt::Debug,
{
    match AsBinding::binds(prefix, extender_variables.0) {
        None => match AsBinding::binds(prefix, extender_variables.1) {
            None => {
                error!(
                    "Neither extender variable {:?} bound by prefix {:?}.",
                    extender_variables, prefix
                );
                Err("Neither extender variable bound by prefix.")
            }
            Some(offset) => Ok(Direction::Reverse(offset)),
        },
        Some(offset) => {
            match AsBinding::binds(prefix, extender_variables.1) {
                Some(_) => Err("Both extender variables already bound by prefix."),
                None => {
                    // Prefix binds the first extender variable, but not
                    // the second. Can use forward index.
                    Ok(Direction::Forward(offset))
                }
            }
        }
    }
}

/// Bindings can be in conflict with the source binding of a given
/// delta pipeline. We need to identify them and handle them as