code-example.rs

mod enum_mod {
    export run;

    enum expr {
        val(int),
        plus(&expr, &expr),
        minus(&expr, &expr)
    }
    
    fn eval(e: &expr) -> int {
        alt *e {
          val(i) => i,
          plus(a,b) => eval(a) + eval(b),
          minus(a,b) => eval(a) - eval(b)
        }
    }
    
    fn run() {
        let x = eval(

            &minus(&val(5),
                   &plus(&val(3), &val(1))));
        
        io::println(#fmt("val: %i", x));
    }
}

trait Expr {
    fn eval() -> int;
}


enum plus<T:Expr,T2:Expr> = 
    (T,T2);
enum minus<T:Expr,T2:Expr> = 
    (T,T2);


impl plus_expr<T:Expr,T2:Expr> of Expr for plus<T,T2> {
    fn eval() -> int {
        alt self {
          plus((ref a, ref b)) => a.eval() + b.eval()
        }
    }
}


impl minus_expr<T:Expr,T2:Expr> of Expr for minus<T,T2> {
    fn eval() -> int {
        alt self {
          minus((ref a, ref b)) => a.eval() - b.eval()
        }
    }
}

impl of Expr for int {
    fn eval() -> int { self }
}


fn main() {
    enum_mod::run();    
    let v = minus((5, plus((3, 1))));
    io::println(#fmt("%i", v.eval()));

}