my first rust program: a really crappy todo list

todo.rs

use std::io::buffered::BufferedReader;
use std::io;

#[deriving(Clone)]
struct Todo {
    desc: ~str,
    completed: bool
}

impl Todo {
    fn new(desc: ~str) -> Todo {
        Todo {desc: desc, completed: false}
    }

    fn completed(&self) -> Todo {
        Todo {desc: self.desc.clone(), completed: true}
    }
}

struct TodoList {
    todos: ~[Todo]
}

impl TodoList {
    fn new() -> TodoList {
        TodoList {todos: ~[]}
    }
    fn add(&self, todo: Todo) -> TodoList {
        TodoList {todos: std::vec::append_one(self.todos.clone(), todo)}
    }
    fn complete(&self, idx: uint) -> TodoList {
        let mut ntodos = self.todos.clone();
        ntodos[idx] = ntodos[idx].completed();
        TodoList {todos: ntodos}
    }
    fn clear(&self) -> TodoList {
        let (_, uncompleted) = self.todos.partitioned({|todo| todo.completed});
        TodoList {todos: uncompleted}
    }
    fn display(&self) {
        for (i, todo) in self.todos.iter().enumerate() {
            println!("{}.\t{:?}", i, todo);
        }
    }
}

struct AddOp;
struct CompleteOp;
struct ClearOp;

impl AddOp {
    fn execute(todos: &TodoList, desc: &str) -> TodoList {
        todos.add(Todo::new(desc.into_owned()))
    }
}

impl CompleteOp {
    fn execute(todos: &TodoList, idx: uint) -> TodoList {
        todos.complete(idx)
    }
}

impl ClearOp {
    fn execute(todos: &TodoList) -> TodoList {
        todos.clear()
    }
}

fn main() {
    let mut todos = TodoList::new();
    let mut reader = BufferedReader::new(io::stdin());

    println!("Welcome to the Todo List!");

    loop {
        println!("Current Items:");
        todos.display();
        println!("What would you like to do?");
        println!("\tADD <desc>\t\tAdd new todo item");
        println!("\tCOMPLETE <index>\tMark selected item completed");
        println!("\tCLEAR\t\t\tRemove completed items");
        println!("\tEXIT\t\t\tExit the program")
        // TODO: figure out how to print command prompt
        let input = reader.read_line().unwrap_or(~"nothing");
        todos = match input.split(' ').nth(0) {
            Some("ADD") => AddOp::execute(&todos, input.slice(4, input.len() - 1)),
            Some("COMPLETE") => CompleteOp::execute(&todos, from_str::<uint>(input.slice(9,10)).unwrap_or(0)),
            Some("CLEAR") => ClearOp::execute(&todos),
            Some("EXIT") => break,
            _ => todos
        };
    }

}