構文木

token_tree.rs

enum Literal { Int(i32), }

enum Term<'a> { OpTree(&'a OpTree<'a>), Literal(Literal) }

enum OpType { Add, Sub, Mul, Div }

struct OpTree<'a>(OpType, Term<'a>, Term<'a>);

impl ToString for OpType {
    fn to_string(&self) -> String {
        use OpType::*;

        match self {
            &Add => "+",
            &Sub => "-",
            &Mul => "*",
            &Div => "/",
        }.into()
    }
}

fn print_literal(lit: &Literal) {
    use Literal::*;

    match lit {
        &Int(i) => { print!("{}", i); }
    }
}

fn print_term(term: &Term) {
    use Term::*;

    match term {
        &OpTree(ref tree) => print_optree(tree),
        &Literal(ref lit) => print_literal(lit),
    }
}

fn print_optree<'a>(tree: &'a OpTree) {
    let &OpTree(ref op, ref lhs, ref rhs) = tree;

    print!("(");
    print_term(&lhs);
    print!("{}", op.to_string());
    print_term(&rhs);
    print!(")");
}

fn main() {
    let tree1 = OpTree(OpType::Add, Term::Literal(Literal::Int(1)), Term::Literal(Literal::Int(2)));
    print_optree(&tree1);
    println!();

    let tree2 = OpTree(OpType::Sub, Term::OpTree(&tree1), Term::Literal(Literal::Int(2)));
    print_optree(&tree2);
    println!();
}