//! This file defines the co-algebra trait.
//!
//! If F is an endo-functor, then an F-co-algebra is a natural
//! transformation from the identity functor to F.

use super::functor::Functor;

/// A co-algebra is a function from T to F(T).
///
/// This is a "trait alias".  Since Rust does not support trait alias
/// yet, we define an empty trait with an automatic implementation.
pub trait Coalgebra<T, F: Functor<T>>: FnMut(T) -> F {}

impl<T, F: Functor<T>, C: FnMut(T) -> F> Coalgebra<T, F> for C {}