Rust组合器

在 Rust 中，组合器更多的是用于对返回结果的类型进行变换：例如使用 ok_or 将一个 Option 类型转换成 Result 类型。

下面我们来看看一些常见的组合器。

or() 和 and()

跟布尔关系的与/或很像，这两个方法会对两个表达式做逻辑组合，最终返回 Option / Result。

• or()，表达式按照顺序求值，若任何一个表达式的结果是 Some 或 Ok，则该值会立刻返回
• and()，若两个表达式的结果都是 Some 或 Ok，则第二个表达式中的值被返回。若任何一个的结果是 None 或 Err ，则立刻返回。

实际上，只要将布尔表达式的 true / false，替换成Some / None或 Ok / Err 就很好理解了。

filter()

filter 用于对 Option 进行过滤。

map() 和 map_err()

map 可以将 Some 或 Ok 中的值映射为另一个。

map_or() 和 map_or_else()

map_or 在 map 的基础上提供了一个默认值。

map_or_else 与 map_or 类似，但是它是通过一个闭包来提供默认值:

ok_or() and ok_or_else()

这两兄弟可以将 Option 类型转换为 Result 类型。其中 ok_or 接收一个默认的 Err 参数

而 ok_or_else 接收一个闭包作为Err 参数

以上列出的只是常用的一部分，强烈建议大家看看标准库中有哪些可用的 API，在实际项目中，这些 API 将会非常有用: Option 和 Result。

详细：https://blog.csdn.net/Unicorn_wan/article/details/125113449

fn or_and() {
    let s1 = Some("some1");
    let s2 = Some("some2");
    let n: Option<&str> = None;
  
    let o1: Result<&str, &str> = Ok("ok1");
    let o2: Result<&str, &str> = Ok("ok2");
    let e1: Result<&str, &str> = Err("error1");
    let e2: Result<&str, &str> = Err("error2");
  
    assert_eq!(s1.or(s2), s1); // Some1 or Some2 = Some1  1、Option or两个有值，返回第一个
    assert_eq!(s1.or(n), s1);  // Some or None = Some    
    assert_eq!(n.or(s1), s1);  // None or Some = Some     2、Option 只要有Some值就返回值
    assert_eq!(n.or(n), n);    // None1 or None2 = None2
  
    assert_eq!(o1.or(o2), o1); // Ok1 or Ok2 = Ok1     1、Result 两个值OK，返回Ok值
    assert_eq!(o1.or(e1), o1); // Ok or Err = Ok       2、一个Ok，一个Err，返回Ok
    assert_eq!(e1.or(o1), o1); // Err or Ok = Ok
    assert_eq!(e1.or(e2), e2); // Err1 or Err2 = Err2   3、两个Err，返回Err
  
    assert_eq!(s1.and(s2), s2); // Some1 and Some2 = Some2  1、Option  and都有值，返回第二个
    assert_eq!(s1.and(n), n);   // Some and None = None     2、有None，就返回None
    assert_eq!(n.and(s1), n);   // None and Some = None
    assert_eq!(n.and(n), n);    // None1 and None2 = None1
  
    assert_eq!(o1.and(o2), o2); // Ok1 and Ok2 = Ok2   1、 Result 都OK返回第二个
    assert_eq!(o1.and(e1), e1); // Ok and Err = Err    2、有Err返回Err
    assert_eq!(e1.and(o1), e1); // Err and Ok = Err 
    assert_eq!(e1.and(e2), e1); // Err1 and Err2 = Err1
}


fn or_else() {
    // or_else with Option
    let s1 = Some("some1");
    let s2 = Some("some2");
    let fn_some = || Some("some2"); // 类似于: let fn_some = || -> Option<&str> { Some("some2") };

    let n: Option<&str> = None;
    let fn_none = || None;

    assert_eq!(s1.or_else(fn_some), s1);  // Some1 or_else Some2 = Some1  1、Option  or_else都有Some值，返回第一个
    assert_eq!(s1.or_else(fn_none), s1);  // Some or_else None = Some      2、 有一个Some值就返回值
    assert_eq!(n.or_else(fn_some), s2);   // None or_else Some = Some
    assert_eq!(n.or_else(fn_none), None); // None1 or_else None2 = None2

    // or_else with Result
    let o1: Result<&str, &str> = Ok("ok1");
    let o2: Result<&str, &str> = Ok("ok2");
    let fn_ok = |_| Ok("ok2"); // 类似于: let fn_ok = |_| -> Result<&str, &str> { Ok("ok2") };

    let e1: Result<&str, &str> = Err("error1");
    let e2: Result<&str, &str> = Err("error2");
    let fn_err = |_| Err("error2");

    assert_eq!(o1.or_else(fn_ok), o1);  // Ok1 or_else Ok2 = Ok1  1、Result 有Ok值返回Ok值，两个相同返回第一个
    assert_eq!(o1.or_else(fn_err), o1); // Ok or_else Err = Ok    
    assert_eq!(e1.or_else(fn_ok), o2);  // Err or_else Ok = Ok
    assert_eq!(e1.or_else(fn_err), e2); // Err1 or_else Err2 = Err2
}

fn or_then() {
        // and_then with Option
        let s1 = Some("some1");
        let s2 = Some("some2");
        let fn_some = |_| Some("some2"); // 类似于: let fn_some = |_| -> Option<&str> { Some("some2") };
    
        let n: Option<&str> = None;
        let fn_none = |_| None;
    
        assert_eq!(s1.and_then(fn_some), s2); // Some1 and_then Some2 = Some2  1、Option  两个Some值返回最后一个
        assert_eq!(s1.and_then(fn_none), n);  // Some and_then None = None  2、只要有None，就返回None
        assert_eq!(n.and_then(fn_some), n);   // None and_then Some = None
        assert_eq!(n.and_then(fn_none), n);   // None1 and_then None2 = None1
    
        // and_then with Result
        let o1: Result<&str, &str> = Ok("ok1");
        let o2: Result<&str, &str> = Ok("ok2");
        let fn_ok = |_| Ok("ok2"); // 类似于: let fn_ok = |_| -> Result<&str, &str> { Ok("ok2") };
    
        let e1: Result<&str, &str> = Err("error1");
        let e2: Result<&str, &str> = Err("error2");
        let fn_err = |_| Err("error2");
    
        assert_eq!(o1.and_then(fn_ok), o2);  // Ok1 and_then Ok2 = Ok2
        assert_eq!(o1.and_then(fn_err), e2); // Ok and_then Err = Err
        assert_eq!(e1.and_then(fn_ok), e1);  // Err and_then Ok = Err
        assert_eq!(e1.and_then(fn_err), e1); // Err1 and_then Err2 = Err1
}

fn filter_() {
    // filter 用于对 Option 进行过滤, 返回Option
    let s1 = Some(3);
    let s2 = Some(6);
    let n = None;

    let fn_is_even = |x: &i8| x % 2 == 0;

    assert_eq!(s1.filter(fn_is_even), n);  // Some(3) -> 3 is not even -> None
    assert_eq!(s2.filter(fn_is_even), s2); // Some(6) -> 6 is even -> Some(6)
    assert_eq!(n.filter(fn_is_even), n);   // None -> no value -> None
}

fn map_() {
    //map 可以将 Some 或 Ok 中的值映射为另一个
    let s1 = Some("abcde");
    let s2 = Some(5);

    let n1: Option<&str> = None;
    let n2: Option<usize> = None;

    let o1: Result<&str, &str> = Ok("abcde");
    let o2: Result<usize, &str> = Ok(5);

    let e1: Result<&str, &str> = Err("abcde");
    let e2: Result<usize, &str> = Err("abcde");

    let fn_character_count = |s: &str| s.chars().count();

    assert_eq!(s1.map(fn_character_count), s2); // Some1 map = Some2
    assert_eq!(n1.map(fn_character_count), n2); // None1 map = None2

    assert_eq!(o1.map(fn_character_count), o2); // Ok1 map = Ok2
    assert_eq!(e1.map(fn_character_count), e2); // Err1 map = Err2
}


//但是如果你想要将 Err 中的值进行改变， map 就无能为力了，此时我们需要用 map_err
fn map_err_() {
    let o1: Result<&str, &str> = Ok("abcde");
    let o2: Result<&str, isize> = Ok("abcde");

    let e1: Result<&str, &str> = Err("404");
    let e2: Result<&str, isize> = Err(404);

    let fn_character_count = |s: &str| -> isize { s.parse().unwrap() }; // 该函数返回一个 isize

    assert_eq!(o1.map_err(fn_character_count), o2); // Ok1 map = Ok2
    assert_eq!(e1.map_err(fn_character_count), e2); // Err1 map = Err2
}

fn map_or_map_or_else() {
    const V_DEFAULT: u32 = 1;

    let s: Result<u32, ()> = Ok(10);
    let n: Option<u32> = None;
    let fn_closure = |v: u32| v + 2;

    assert_eq!(s.map_or(V_DEFAULT, fn_closure), 12);
    assert_eq!(n.map_or(V_DEFAULT, fn_closure), V_DEFAULT);


    let s = Some(10);
    let n: Option<i8> = None;

    let fn_closure = |v: i8| v + 2;
    let fn_default = || 1;

    assert_eq!(s.map_or_else(fn_default, fn_closure), 12);
    assert_eq!(n.map_or_else(fn_default, fn_closure), 1);

    //map_or_else 与 map_or 类似，但是它是通过一个闭包来提供默认值

    let o = Ok(10);
    let e = Err(5);
    let fn_default_for_result = |v: i8| v + 1; // 闭包可以对 Err 中的值进行处理，并返回一个新值

    assert_eq!(o.map_or_else(fn_default_for_result, fn_closure), 12);
    assert_eq!(e.map_or_else(fn_default_for_result, fn_closure), 6);
}

fn ok_or_else() {
    const ERR_DEFAULT: &str = "error message";

    let s = Some("abcde");
    let n: Option<&str> = None;

    let o: Result<&str, &str> = Ok("abcde");
    let e: Result<&str, &str> = Err(ERR_DEFAULT);

    assert_eq!(s.ok_or(ERR_DEFAULT), o); // Some(T) -> Ok(T)
    assert_eq!(n.ok_or(ERR_DEFAULT), e); // None -> Err(default)

    //而 ok_or_else 接收一个闭包作为 Err 参数:
    let s = Some("abcde");
    let n: Option<&str> = None;
    let fn_err_message = || "error message";

    let o: Result<&str, &str> = Ok("abcde");
    let e: Result<&str, &str> = Err("error message");

    assert_eq!(s.ok_or_else(fn_err_message), o); // Some(T) -> Ok(T)
    assert_eq!(n.ok_or_else(fn_err_message), e); // None -> Err(default)
}

fn main() {

    or_and();
    or_then();
    or_else();

    filter_();

    map_();
    map_err_();
    map_or_map_or_else();
}