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();
}