algar 0.4.1

Algebraic structures, higher-kinded types and other category theory bad ideas
Documentation 
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// data Expr = Val Int | Add Expr Expr
// eval :: Expr -> Int
// eval (Val x) = x
// eval (Add l r) = eval l + eval r
#![allow(dead_code)]

pub enum Expr {
    Val(i32),
    Add(Box<Expr>, Box<Expr>),
}

impl Expr {
    fn eval(expr: Expr) -> i32 {
        match expr {
            Expr::Val(i) => i,
            Expr::Add(x, y) => Self::eval(*x) + Self::eval(*y),
        }
    }

    fn render(expr: Expr) -> String {
        match expr {
            Expr::Val(i) => i.to_string(),
            Expr::Add(x, y) => format!("{} + {}", Self::render(*x), Self::render(*y)),
        }
    }
}

pub fn int_val<E: From<Expr>>(value: i32) -> E {
    E::from(Expr::Val(value))
}

pub fn add<E: From<Expr>>(lhs: Expr, rhs: Expr) -> E {
    E::from(Expr::Add(Box::new(lhs), Box::new(rhs)))
}

fn main() {}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn simple_eval_expression() {
        let expr = add(int_val(1), int_val(2));

        assert_eq!(3, Expr::eval(expr))
    }

    #[test]
    fn simple_render_expression() {
        let expr = add(int_val(1), int_val(2));

        assert_eq!("1 + 2", Expr::render(expr))
    }
}