gistfile1.txt

enum Expression{
    Number(int), 
    Add(~Expression,~Expression),
    Sub(~Expression,~Expression),
    
    
}

fn eval(e: Expression) -> int {
    match e {
        Add(x,y) => {eval(*x) + eval(*y)}
        Sub(x,y) => {eval(*x) - eval(*y)}
        Number(x) => {x}
        
    }
}
fn main() {

    let x = Add(~Number(5),~Number(10));
    let result = eval(x);

    println(fmt!("%?",result));
}

enum Expression{
  Number(int),
  Add(~Expression,~Expression),
  Sub(~Expression,~Expression),
}

fn eval(e: &Expression) -> int {
  match *e {
    Add(ref x,ref y) => {eval(*x) + eval(*y)}
    Sub(ref x,ref y) => {eval(*x) - eval(*y)}
    Number(x) => {x}
  }
}
fn main() {

  let x = Add(~Number(5),~Number(10));
  let result = eval(&x);

  println(fmt!("%?",result));
}

Does this work?

enum Expression{
    Number(int), 
    Add(~Expression,~Expression),
    Sub(~Expression,~Expression),


}

fn eval(e: Expression) -> int {
    match e {
        Add(~x,~y) => {eval(x) + eval(y)}
        Sub(~x,~y) => {eval(x) - eval(y)}
        Number(x) => {x}

    }
}
fn main() {

    let x = Add(~Number(5),~Number(10));
    let result = eval(x);

    println(fmt!("%?",result));
}