//! This file provides a default implementation of the
//! [`Chain`][crate::Chain] trait.
//!
//! The reason for using a trait is that I might want to experiment
//! with different implementation ideas in the future, and this
//! modular design makes that easy.

use super::*;
use core::fmt::Display;

/// The errors related to taking derivatives by chain rule.
#[derive(Debug)]
pub enum Error {
    /// An invalid situation happens.
    Invalid,
}

impl Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Invalid => write!(f, "invalid"),
        }
    }
}

impl std::error::Error for Error {}

/// A default implementation for the [`Chain`] trait.
#[derive(Debug, Clone, Default)]
pub struct DefaultChain {}

impl Chain for DefaultChain {
    type Error = Error;

    fn unit() -> Self {
        todo!()
    }

    fn chain(&mut self, _t: usize) {
        todo!()
    }

    fn epsilon(&self) -> bool {
        todo!()
    }
}