我可以强制性状是协变的吗? [英] Can I force a trait to be covariant?
问题描述
Thanks to @francis-gagné 's excellent answer to another question, I have a clearer view of how variance works. For example, a type containing a reference is covariant over its lifetime parameter, as demonstrated below.
struct Foo<'a> (PhantomData<&'a str>);
/// Foo is covariant over its lifetime parameter
pub fn test_foo<'a:'b, 'b:'c, 'c>() {
let fa: Foo<'a> = Foo(PhantomData);
let fb: Foo<'b> = Foo(PhantomData);
let fc: Foo<'c> = Foo(PhantomData);
let v: Vec<Foo<'b>> = vec![fa, fb]; // fc is not accepted
}
另一方面,接受引用(或包含引用的类型)的函数与其生命周期参数相反.
On the other hand a function accepting a reference (or a type containing it) is contravariant over its lifetime parameter.
struct Bar<'a> (PhantomData<fn(&'a str)>);
/// Bar is contravariant over its lifetime parameter
pub fn test_bar<'a:'b, 'b:'c, 'c>() {
let ba: Bar<'a> = Bar(PhantomData);
let bb: Bar<'b> = Bar(PhantomData);
let bc: Bar<'c> = Bar(PhantomData);
let v: Vec<Bar<'b>> = vec![bb, bc]; // ba is not accepted
}
最后,具有生存期参数的 trait 在其生存期参数上是不变的.
Finally, a trait with a lifetime parameter is invariant over its lifetime parameter.
pub trait Baz<'a> {}
impl<'a> Baz<'a> for () {}
/// Baz is invariant over its lifetime parameter
pub fn test_baz<'a:'b, 'b:'c, 'c>() {
let za: Box<dyn Baz<'a>> = Box::new(());
let zb: Box<dyn Baz<'b>> = Box::new(());
let zc: Box<dyn Baz<'c>> = Box::new(());
let v: Vec<Box<dyn Baz<'b>>> = vec![zb]; // za and zx are not accepted
}
这是有道理的,因为特征既可以通过协变类型也可以通过反变量类型实现,如下所示.
That makes sense, because the trait could be implemented both by a covariant and a contravariant type, as illustrated below.
impl<'a> Baz<'a> for Foo<'a> {}
impl<'a> Baz<'a> for Bar<'a> {}
我的问题是:我可以强制某个特征在其生存期参数上进行协变吗?我希望有一个标记特征,例如:
My question is: can I force a trait to be covariant over its lifetime parameter? I would expect a marker trait such as:
trait Baz<'a>: Covariant<'a> {}
这将使使用逆变类型实现该特征成为非法,并允许 za
成为 test_baz 中向量
v
的成员code>以上功能.
that would make it illegal to implement that trait with a contravariant type, and allow za
to be a member of the vector v
in the test_baz
function above.
当然,能够做相反的事情(迫使一个性状成为逆变)也可能是有用的...
Of course, being able to do the opposite (force a trait to be contravariant) could be useful as well...
推荐答案
否.
您可以表达为 any 'x
实现Baz<'x> 的值":
You can express "a value that implements Baz<'x>
for any 'x
":
pub fn test_baz<'a:'b, 'b:'c, 'c>() {
let za: Box<dyn for<'x> Baz<'x>> = Box::new(());
let zb: Box<dyn for<'x> Baz<'x>> = Box::new(());
let zc: Box<dyn for<'x> Baz<'x>> = Box::new(());
let v: Vec<Box<dyn for<'x> Baz<'x>>> = vec![za, zb, zc];
}
但是(从Rust 1.31开始)您不能为 Box< dyn.Baz''x>>
,即使您可以,该语法也只能使用一生.它不会让您表达类型参数的协方差.
But you can't (as of Rust 1.31) write Box<dyn for<'x: 'b> Baz<'x>>
, and even if you could, that syntax would only work for lifetimes; it wouldn't let you express covariance over type parameters.
这篇关于我可以强制性状是协变的吗?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!