p_rust 0.1.0

My rust practice
Documentation 
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use std::{error, fs};
use std::fs::File;
use std::io::{ErrorKind, Error, Read};
use std::net::IpAddr;

fn read_username_from_file() -> Result<String, Error> {
    let mut f = match File::open("hello.txt") {
        Ok(file) => file,
        Err(e) => return Err(e)
    };

    let mut s = String::new();

    match f.read_to_string(&mut s) {
        Ok(_) => Ok(s),
        Err(e) => Err(e)
    }
}

fn read_username_from_file2() -> Result<String, Error> {
    let mut f = File::open("hello.txt")?;
    let mut s = String::new();
    f.read_to_string(&mut s)?;
    Ok(s)
}

fn read_username_from_file3() -> Result<String, Error> {
    let mut s = String::new();
    File::open("hello.txt")?.read_to_string(&mut s)?;
    Ok(s)
}

fn read_username_from_file4() -> Result<String, Error> {
    fs::read_to_string("hello.txt")
}

fn last_char_of_first_line(text: &str) -> Option<char> {
    text.lines().next()?.chars().last()
}

fn main() -> Result<(), Box<dyn error::Error>> {
    // panic!("crash and burn");

    // let v = vec![1, 2, 3];
    // v[99];

    // let f: u32 = File::open("hello.txt");

    let f = File::open("hello.txt");
    let f = match f {
        Ok(file) => file,
        Err(error) => match error.kind() {
            ErrorKind::NotFound => match File::create("hello.txt") {
                Ok(fc) => fc,
                Err(e) => panic!("Problems opening the file: {:?}", e)
            },
            other_error => panic!("Problems opening the file: {:#?}", other_error)
        }
    };
    println!("f: {:#?}", f);

    let f = File::open("hello.txt").unwrap_or_else(|error| {
        if error.kind() == ErrorKind::NotFound {
            File::create("hello.txt").unwrap_or_else(|error| {
                panic!("Problem creating the file: {:?}", error);
            })
        } else {
            panic!("Problem opening the file: {:?}", error);
        }
    });
    println!("f: {:#?}", f);

    let result = read_username_from_file();
    println!("result: {:?}", result.unwrap());

    let result = read_username_from_file2();
    println!("result: {:?}", result.unwrap());

    let result = read_username_from_file3();
    println!("result: {:?}", result.unwrap());

    let result = read_username_from_file4();
    println!("result: {:?}", result.unwrap());

    last_char_of_first_line("abcde");

    let home: IpAddr = "127.0.0.1".parse().unwrap();
    println!("home: {}", home);

    File::open("hello.txt")?;
    Ok(())
}