Shared via Rust Playground

playground.rs

enum Expression {
    Literal(u32),
    Add(Box<Expression>, Box<Expression>),
    Sub(Box<Expression>, Box<Expression>)
}

impl Expression {
    fn map<F: Fn(Expression) -> Expression>(self, f: F) -> Expression {
        match self {
            Expression::Literal(_) => self,
            Expression::Add(a, b) => Expression::Add(Box::new(f(*a)), Box::new(f(*b))),
            Expression::Sub(a, b) => Expression::Sub(Box::new(f(*a)), Box::new(f(*b)))
        }
    }
}

fn main() {}