Promise type for rust

promise.rs

#[derive(PartialEq)]
pub enum State {
    Pending,
    Resolved,
    Rejected,
}

pub struct Promise<T, TErr> {
    value: Option<Result<T, TErr>>,
    state: State,
}

impl<T, TErr> Promise<T, TErr> {
    pub fn new() -> Promise<T, TErr> {
        return Promise {
            state: State::Pending,
            value: None,
        };
    }

    pub fn is_resolved(&self) -> bool {
        return self.state == State::Resolved;
    }

    pub fn is_rejected(&self) -> bool {
        return self.state == State::Rejected;
    }

    pub fn is_pending(&self) -> bool {
        return self.state == State::Pending;
    }

    pub fn resolve(&mut self, value: T) -> bool {
        if !self.is_pending() {
            return false;
        }
        self.value = Some(Ok(value));
        self.state = State::Resolved;
        return true;
    }

    pub fn reject(&mut self, err: TErr) -> bool {
        if !self.is_pending() {
            return false;
        }
        self.value = Some(Err(err));
        self.state = State::Rejected;
        return true;
    }
}