[英]Rust Lifetimes in Higher Order Functions
I have a map: HashMap<&str, Vec<&str>>
and I'm trying to create a reverse lookup map HashMap<&str, &str>
which refers from each of the element in the original vector to the original key.我有一个 map: HashMap<&str, Vec<&str>>
并且我正在尝试创建一个反向查找 map HashMap<&str, &str>
,它从原始向量中的每个元素指向原始键。
Eg map like below例如 map 如下
{
"k": vec!["a", "b"]
}
would be transformed into会变成
{
"a": "k",
"b": "k",
}
It works perfectly fine when I do that with:当我这样做时,它工作得很好:
let substitutes: HashMap<&str, Vec<&str>> = vec![("a", vec!["b", "c"])].into_iter().collect();
// Below works fine
let mut reversed: HashMap<&str, &str> = HashMap::new();
for (&k, v) in substitutes.iter() {
for vv in v.iter() {
reversed.insert(vv, k);
}
}
However it doesn't work if I try to do that with a Higher Order Functions:但是,如果我尝试使用高阶函数来做到这一点,它就不起作用:
// Below doesn't work
let reversed_2: HashMap<&str, &str> = substitutes
.iter()
.flat_map(|(&k, v)| v.iter().map(|&vv| (vv, k)))
.collect();
And gives the following errors:并给出以下错误:
error[E0373]: closure may outlive the current function, but it borrows `k`, which is owned by the current function
--> src/main.rs:18:42
|
18 | .flat_map(|(&k, v)| v.iter().map(|&vv| (vv, k)))
| ^^^^^ - `k` is borrowed here
| |
| may outlive borrowed value `k`
|
note: closure is returned here
--> src/main.rs:18:29
|
18 | .flat_map(|(&k, v)| v.iter().map(|&vv| (vv, k)))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `k` (and any other referenced variables), use the `move` keyword
|
18 | .flat_map(|(&k, v)| v.iter().map(move |&vv| (vv, k)))
| ^^^^^^^^^^
error: aborting due to previous error; 1 warning emitted
I am trying to wrap my head around how vv
might outlive k
given that it's in the same scope under flat_map
.鉴于 vv 在 flat_map 下的同一个 scope 中,我正试图围绕vv
如何比k
更flat_map
。
Would be really helpful to get more information why my HOF approach fails.获得更多关于我的 HOF 方法失败的信息将非常有帮助。
In this case, the compiler help was exactly right: you need to add move
on the inner closure.在这种情况下,编译器的帮助是完全正确的:您需要在内部闭包上添加move
。
let reversed_2: HashMap<&str, &str> = substitutes
.iter()
.flat_map(|(&k, v)| v.iter().map(move |&vv| (vv, k)))
.collect();
When you make an anonymous function that uses variables from the surrounding scope, the inline function (called a closure ) needs access to those variables.当您使用来自周围 scope 的变量创建匿名 function 时,内联 function(称为闭包)需要访问这些变量。 By default, it takes a reference to them, leaving them available in the enclosing scope.默认情况下,它引用它们,使它们在封闭的 scope 中可用。 However, in this case, the closure takes a reference to k
and returns it, which means that it could escape the enclosing scope.但是,在这种情况下,闭包会引用k
并返回它,这意味着它可以逃脱封闭的 scope。
.flat_map(|(&k, v)| {
// k is only valid in this anonymous function
// a reference to k is returned out of map and also out of flat_map
// (Note: no move)
v.iter().map(|&vv| (vv, k))
// the anonymous function ends, all references to k need to have ended
})
// A reference to k has been returned, even though k's lifetime has ended
// Compiler error!
When you switch to a move
closure, rather than taking to references to things in the enclosing environment, you take ownership of the things themselves.当您切换到move
闭包时,您无需引用封闭环境中的事物,而是获得事物本身的所有权。 This lets you return k
rather than a reference to k
, sidestepping the issue.这使您可以返回k
而不是对k
的引用,从而回避了这个问题。
.flat_map(|(&k, v)| {
// k is only valid in this anonymous function
// k is moved out of the enclosing function and into the inner function
// Because it owns k, it can return it
// (Note: uses move)
v.iter().map(move |&vv| (vv, k))
// k has been moved out of this environment, so its lifetime does not end here
})
// k has been returned and is still alive
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