mod exp {
    pub trait Exp {
        fn lit(_: Self) -> Self;
        fn neg(_: Self) -> Self;
        fn add(_: Self, _: Self) -> Self;
    }

    pub fn lit<T: Exp>(x: T) -> T {
        T::lit(x)
    }

    pub fn neg<T: Exp>(x: T) -> T {
        T::neg(x)
    }

    pub fn add<T: Exp>(x: T, y: T) -> T {
        T::add(x, y)
    }

    // view et eval choisissent l'impl en imposant le type du paramètre
    pub fn view<T: Exp>(x: T) -> T {
        x
    }
    pub fn eval<T: Exp>(x: T) -> T {
        x
    }

    impl Exp for i32 {
        fn lit(x: Self) -> Self {
            x
        }
        fn neg(x: Self) -> Self {
            -x
        }
        fn add(e1: Self, e2: Self) -> Self {
            e1 + e2
        }
    }

    impl Exp for String {
        fn lit(x: Self) -> Self {
            format!("{}", x)
        }
        fn neg(x: Self) -> Self {
            format!("- {}", x)
        }
        fn add(e1: Self, e2: Self) -> Self {
            format!("{} + {}", e1, e2)
        }
    }
}

use exp::*;

fn main() {
    let x = add(lit(5), neg(lit(3)));
    println!("{}", view(x));
    println!("{}", eval(x));
}