add_sub.rs

enum Expr {
  Num(i64),
  Op(Box<Expr>, fn(i64, i64) -> i64, Box<Expr>),
}

#[derive(Debug)]
enum Token {
  Num(i64),
  Add,
  Sub,
}

impl Expr {
  fn from(tokens: &[Token]) -> Result<Expr, String> {
    match tokens {
      &[Token::Num(l), Token::Add, ref r @ ..] => Ok(Expr::Op(
        Box::new(Expr::Num(l)),
        |a, b| a + b,
        Box::new(Expr::from(r)?)
      )),
      &[Token::Num(l), Token::Sub, ref r @ ..] => Ok(Expr::Op(
        Box::new(Expr::Num(l)),
        |a, b| a - b,
        Box::new(Expr::from(r)?)
      )),
      &[Token::Num(n)] => Ok(Expr::Num(n)),
      _ => Err(format!("syntax error: {:#?}", tokens)),
    }
  }

  fn eval(&self) -> i64 {
    match self {
      Expr::Num(n) => *n,
      Expr::Op(l, f, r) => f(l.eval(), r.eval()),
    }
  }
}

fn main() {
  let expr = Expr::from(&[
    Token::Num(1),
    Token::Add,
    Token::Num(2),
    Token::Sub,
    Token::Num(-4)
  ]).unwrap();
  println!("{}", expr.eval());
}