fifo queue taking input from stdin

fifo_queue_stdin.rs

use std::io;

struct Node {
    value: String,
    next: Option<Box<Node>>
}

struct List {
    head: Option<Box<Node>>
}

impl Node {
    fn attach_new(&mut self, val: String) {
        match self.next {
            Some (ref mut next) => next.attach_new(val),
            None => self.next = Some(Box::new(Node {value: val, next: None})) 
        }
    }
}

impl List {
    fn push_back(&mut self, val: String) {
        match self.head {
            Some(ref mut node) => node.attach_new(val),
            None => self.head = Some(Box::new(Node {value: val, next: None}))
        }
    }

    fn pull_front(&mut self) -> String {
        if self.head.is_some() {
            let detached_head = self.head.take().unwrap();
            let ret_val = detached_head.value.clone();
            self.head = detached_head.next;

            return ret_val;
        } else {
            return "".to_string();
        }
    }
}

fn input_reader(list: &mut List, input: String) -> Option<String> {
    if input == "POP" {
        return Some(list.pull_front());
    } else {
        let pieces: Vec<&str> = input.split(" ").collect();
        if pieces[0] == "PUSH" {
            list.push_back(pieces[1].to_string());
        }
        return None;
    }
}

fn main() {
    let mut queue = List {head: None};

    let mut input = String::new();

    loop {
        input = "".to_string();
        io::stdin().read_line(&mut input);
        input = input.trim().to_string();

        if input.len() > 0 {
            let s = input_reader(&mut queue, input);
            match s {
                Some(res) => println!("> {}", res),
                None => continue
            };
        }
    }
}