如何实现一个带有特征 MyTrait<A> 的结构？

Question

我定义了一个特征如下：

trait Readable<E> {
    fn read_u8(&mut self) -> Result<u8, E>;
    fn read_u16be(&mut self) -> Result<u16, E>;
}

想法是用不同的后端返回不同的错误类型来实现它。我试图在函数中使用它：

fn f<E, R: Readable<E>>(r: &mut R) -> Result<u8, E> {
    r.read_u8()
}

编译通过。我在 impl 中尝试了同样的方法：

struct FileFormat<R> {
    r: R,
}

impl<E, R: Readable<E>> FileFormat<R> {
    fn f(&mut self) -> Result<u8, E> {
        self.r.read_u8()
    }
}

Playground

这失败了：

14 | impl<E, R: Readable<E>> FileFormat<R> {
   |      ^ unconstrained type parameter

编译器建议 rustc --explain E0207 但恐怕我无法理解其中包含的答案。

为什么前者可以编译而后者不能？为什么 E 在这种情况下不受约束？如何解决这个问题，以便实施能够采取任何 Readable?

Answer 1

让我们想象一下它确实编译了。这是一个注释示例，表明它可以让我们编写编译器无法进行类型检查的损坏代码：

trait Readable<E> {
    fn read_u8(&mut self) -> Result<u8, E>;
    fn read_u16be(&mut self) -> Result<u16, E>;
}

fn f<E, R: Readable<E>>(r: &mut R) -> Result<u8, E> {
    r.read_u8()
}

struct SomeError;
struct SomeOtherError;
struct SomeReadable;

impl Readable<SomeError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeError> {
        todo!()
    }
}

impl Readable<SomeOtherError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeOtherError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeOtherError> {
        todo!()
    }
}

struct FileFormat<R> {
    r: R,
}

// let's pretend that this does compile
impl<E, R: Readable<E>> FileFormat<R> {
    fn f(&mut self) -> Result<u8, E> {
        self.r.read_u8()
    }
}

// it will now allow us to write this code
// which is impossible to type check so
// it's obviously broken
fn example(mut fr: FileFormat<SomeReadable>) -> Result<u8, ???> {
    // um, does this return Result<u8, SomeError>
    // or does it return Result<u8, SomeOtherError>???
    // it's impossible to know!
    fr.f()
}

playground

错误类型需要出现在 FileFormat 类型中的某处。修复就像将 PhantomData 成员添加到 FileFormat 一样简单，这样您就可以“确定”特定的错误类型：

use core::marker::PhantomData;
trait Readable<E> {
    fn read_u8(&mut self) -> Result<u8, E>;
    fn read_u16be(&mut self) -> Result<u16, E>;
}

fn f<E, R: Readable<E>>(r: &mut R) -> Result<u8, E> {
    r.read_u8()
}

struct SomeError;
struct SomeOtherError;
struct SomeReadable;

impl Readable<SomeError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeError> {
        todo!()
    }
}

impl Readable<SomeOtherError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeOtherError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeOtherError> {
        todo!()
    }
}

struct FileFormat<R, E> {
    r: R,
    e: PhantomData<E>,
}

// now compiles!
impl<E, R: Readable<E>> FileFormat<R, E> {
    fn f(&mut self) -> Result<u8, E> {
        self.r.read_u8()
    }
}

// now works!
fn example(mut fr: FileFormat<SomeReadable, SomeError>) -> Result<u8, SomeError> {
    fr.f()
}

// now also works!
fn other_example(mut fr: FileFormat<SomeReadable, SomeOtherError>) -> Result<u8, SomeOtherError> {
    fr.f()
}

playground

独立的通用函数可以工作，因为我们在调用该函数时指定了错误类型：

trait Readable<E> {
    fn read_u8(&mut self) -> Result<u8, E>;
    fn read_u16be(&mut self) -> Result<u16, E>;
}

struct SomeError;
struct SomeOtherError;
struct SomeReadable;

impl Readable<SomeError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeError> {
        todo!()
    }
}

impl Readable<SomeOtherError> for SomeReadable {
    fn read_u8(&mut self) -> Result<u8, SomeOtherError> {
        todo!()
    }
    fn read_u16be(&mut self) -> Result<u16, SomeOtherError> {
        todo!()
    }
}

fn f<E, R: Readable<E>>(r: &mut R) -> Result<u8, E> {
    r.read_u8()
}

fn example() {
    let mut readable: SomeReadable = SomeReadable;
    // error type clarified to be SomeError here
    f::<SomeError, _>(&mut readable);

    let mut readable: SomeReadable = SomeReadable;
    // error type clarified to be SomeOtherError here
    f::<SomeOtherError, _>(&mut readable);
}

playground

归根结底就是让你的类型对编译器可见。

如何实现一个带有特征 MyTrait<A> 的结构？

How do implement a struct that takes a trait MyTrait<A>?

generics

polymorphism

traits

rust

parametric-polymorphism