use std::rc::Rc;
#[derive(Debug)]
pub enum StackError {
NoSuchElementError,
IndexOutOfRangeError,
RcUnwrapError,
}
pub trait Stack<A> {
fn cons(self, value: A) -> Self;
fn head(&self) -> Result<&A, StackError>;
fn tail(&self) -> Rc<Self>;
fn size(&self) -> usize;
fn update(self, index: u32, new_value: A) -> Result<Self, StackError>
where
Self: Sized;
fn get(&self, i: u32) -> Result<&A, StackError>;
}