rust - 允许函数接受 `T` 或任何 `FnMut(T) -> T`

Question

我的目标是编译此代码的最后两行并通过最后一个断言:

struct State {
    string: String
}

impl State {
    fn string<F: FnMut(String) -> String>(mut self, mut f: F) -> Self {
        self.string = f(self.string);
        self
    }
}

fn main() {
    let state = State { string: String::from("foo") };
    assert_eq!(state.string, "foo");
    let state = state.string(|old| old + "bar");
    assert_eq!(state.string, "foobar");
    // let state = state.string(String::from("baz"));
    // assert_eq!(state.string, "baz");
}

我认为这可以通过 traits 和 specialization 来实现，但是下面的代码:

#![feature(specialization)]

trait Get<T> {
    fn get(self, old: T) -> T;
}

impl<T> Get<T> for T {
    default fn get(self, _: T) -> T {
        self
    }
}

impl<T, F> Get<T> for F where F: FnMut(T) -> T {
    fn get(mut self, old: T) -> T {
        self(old)
    }
}

struct State {
    string: String
}

impl State {
    fn string<G: Get<String>>(mut self, g: G) -> Self {
        self.string = g.get(self.string);
        self
    }
}

抛出这个错误(live):

error[E0119]: conflicting implementations of trait `Get<_>`:
  --> <anon>:13:1
   |
13 | impl<T, F> Get<T> for F where F: FnMut(T) -> T {
   | ^
   |
note: conflicting implementation is here:
  --> <anon>:7:1
   |
7  | impl<T> Get<T> for T {
   | ^

error: aborting due to previous error

所以我的问题是，为什么 Get 的第二个实现不比第一个实现更“具体”，并且在当前的稳定版或夜间 Rust 中有什么方法可以让我的原始代码工作吗？

编辑:我知道只为一种类型实现特征是可行的，但我想要一个适用于任何类型的通用解决方案，因为我希望能够将其用于结构的任意字段。

Answer 1

对于您的具体问题，您不需要特化:

struct State {
    string: String,
}

impl State {
    fn string<F>(mut self, mut f: F) -> Self
        where F: Thing
    {
        self.string = f.thing(self.string);
        self
    }
}

trait Thing {
    fn thing(&mut self, s: String) -> String;
}

impl Thing for String {
    fn thing(&mut self, _s: String) -> String {
        self.clone()
    }
}

impl<F> Thing for F
    where F: FnMut(String) -> String
{
    fn thing(&mut self, s: String) -> String {
        (self)(s)
    }
}

fn main() {
    let state = State { string: String::from("foo") };
    assert_eq!(state.string, "foo");
    let state = state.string(|old| old + "bar");
    assert_eq!(state.string, "foobar");
    let state = state.string(String::from("baz"));
    assert_eq!(state.string, "baz");
}

您可能想要要求 FnOnce 或为 &str 实现特征。目前，String 的分配未被使用，导致效率低下。

然后您可以为感兴趣的类型多次实现该特征:

struct State {
    string: String,
    vec: Vec<u8>,
}

impl State {
    fn string<F>(mut self, mut f: F) -> Self
        where F: Thing<String>
    {
        self.string = f.thing(self.string);
        self
    }

    fn vec<F>(mut self, mut f: F) -> Self
        where F: Thing<Vec<u8>>
    {
        self.vec = f.thing(self.vec);
        self
    }
}

trait Thing<T> {
    fn thing(&mut self, s: T) -> T;
}

impl Thing<String> for String {
    fn thing(&mut self, _s: String) -> String {
        self.clone()
    }
}

impl<F> Thing<String> for F
    where F: FnMut(String) -> String
{
    fn thing(&mut self, s: String) -> String {
        (self)(s)
    }
}

impl Thing<Vec<u8>> for Vec<u8> {
    fn thing(&mut self, _s: Vec<u8>) -> Vec<u8> {
        self.clone()
    }
}

impl<F> Thing<Vec<u8>> for F
    where F: FnMut(Vec<u8>) -> Vec<u8>
{
    fn thing(&mut self, s: Vec<u8>) -> Vec<u8> {
        (self)(s)
    }
}

fn main() {
    let state = State { string: String::from("foo"), vec: vec![1] };

    assert_eq!(state.string, "foo");
    let state = state.string(|old| old + "bar");
    assert_eq!(state.string, "foobar");
    let state = state.string(String::from("baz"));
    assert_eq!(state.string, "baz");

    assert_eq!(state.vec, [1]);
    let state = state.vec(|mut old: Vec<u8>| {
        old.push(2);
        old
    });
    assert_eq!(state.vec, [1, 2]);
    let state = state.vec(vec![3]);
    assert_eq!(state.vec, [3]);
}

我相信重复可以由宏来处理:

macro_rules! thing {
    ($t: ty) => {
        impl Thing<$t> for $t {
            default fn thing(&mut self, _val: $t) -> $t {
                self.clone()
            }
        }

        impl<F> Thing<$t> for F
            where F: FnMut($t) -> $t
        {
            fn thing(&mut self, val: $t) -> $t {
                (self)(val)
            }
        }
    }
}

thing!(String);
thing!(Vec<u8>);