让我们想象一下它确实可以编译。这是一个有注释的示例，它显示了它允许我们编写残破的代码，这对于编译器而言是不可能进行类型检查的:
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类型内的某个位置。该修复非常简单，只需将一个
PhantomData成员添加到
FileFormat即可，以便您可以“固定”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独立的泛型函数之所以有效，是因为我们在调用函数时会指定Error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实际上，所有这些仅取决于使您的类型对编译器可见。