fn main() {
     let a = 1; // immutable
    let mut b = 1; //mutable
    assert_eq!(b,1);
//    a = 2; // compile error
    b = 2;
    let c :i32 = 1;
    assert_eq!(a,c);
    assert_eq!(b,2);

    let some_i32 :Option<i32> = Some(3);
    let none_i32 :Option<i32> = None;
    assert!(some_i32.is_some());
    assert!(none_i32.is_none());
    assert_eq!(some_i32.unwrap(),3);
    assert_eq!(none_i32.unwrap_or(1),1);

    let ok_i32: Result<i32,&str> = Ok(3);
    let err_i32: Result<i32,&str> = Err("error");
    assert_eq!(ok_i32.unwrap(),3);
    assert_eq!(err_i32.unwrap_or(1),1);
    assert!(ok_i32.is_ok());
    assert!(err_i32.is_err());

    if let Some(x) = some_i32 {
        println!("{}", x);
    } else {
        println!("None");
    }
    if let Some(x) = none_i32 {
        println!("{}", x);
    } else {
        println!("None");
    }

    match some_i32 {
        Some(x) => println!("{}", x),
        None => println!("None"),
    }

    match ok_i32 {
        Ok(x) => println!("{}", x),
        Err(e) => println!("error info. is: {},", e),
    }
    match err_i32 {
        Ok(x) => println!("{}", x),
        Err(e) => println!("{}", e),
    }

}