以下是为Runnable
实施Fn()
的工作示例(以便我们可以直接将&闭包传递给run_the_runnable
函数):
trait Runnable {
fn run(&self);
}
impl<F> Runnable for F where F: Fn() {
fn run(&self) {
self();
}
}
fn run_the_runnable(runnable: &Runnable) {
runnable.run();
}
fn main() {
// runnable without parameters
struct MyRunnable;
impl Runnable for MyRunnable {
fn run(&self) {
println!("Hello from MyRunnable");
}
}
// from struct instance (WORKS)
run_the_runnable(&MyRunnable);
// from closure (WORKS)
run_the_runnable(&|| {
println!("Hello from run() closure");
});
}
现在,让我们更改可运行的run()
方法以接受参考参数(&i32
),并实施Fn(&i32)
:
trait Runnable {
fn run(&self, x: &i32);
}
impl<F> Runnable for F where F: Fn(&i32) {
fn run(&self, x: &i32) {
self(x);
}
}
fn run_the_runnable(runnable: &Runnable, x: &i32) {
runnable.run(x);
}
fn main() {
// runnable without parameters
struct MyRunnable;
impl Runnable for MyRunnable {
fn run(&self, x: &i32) {
println!("Hello from MyRunnable {}", x);
}
}
let x = 42;
// from struct instance (WORKS)
run_the_runnable(&MyRunnable, &x);
// from closure (DOES NOT WORK)
run_the_runnable(&|x| {
println!("Hello from run(&i32) closure {}", x);
}, &x);
}
传递闭包不再起作用了:
error[E0271]: type mismatch resolving `for<'r> <[closure@<anon>:30:27: 32:10] as std::ops::FnOnce<(&'r i32,)>>::Output == ()`
--> <anon>:30:26
|
30 | run_the_runnable(&|x| {
| __________________________^ starting here...
31 | | println!("Hello from run(&i32) closure {}", x);
32 | | }, &x);
| |_________^ ...ending here: expected bound lifetime parameter , found concrete lifetime
|
= note: concrete lifetime that was found is lifetime '_#63r
= note: required because of the requirements on the impl of `Runnable` for `[closure@<anon>:30:27: 32:10]`
= note: required for the cast to the object type `Runnable`
error[E0281]: type mismatch: the type `[closure@<anon>:30:27: 32:10]` implements the trait `std::ops::Fn<(_,)>`, but the trait `for<'r> std::ops::Fn<(&'r i32,)>` is required (expected concrete lifetime, found bound lifetime parameter )
--> <anon>:30:26
|
30 | run_the_runnable(&|x| {
| __________________________^ starting here...
31 | | println!("Hello from run(&i32) closure {}", x);
32 | | }, &x);
| |_________^ ...ending here
|
= note: required because of the requirements on the impl of `Runnable` for `[closure@<anon>:30:27: 32:10]`
= note: required for the cast to the object type `Runnable`
关于此错误有很多问题:
但我仍然无法解决这个特定的终身问题。
答案 0 :(得分:2)
正如comment中所建议的那样,在闭包参数中给出显式类型可以解决问题:
@@ -26,8 +26,8 @@ fn main() {
// from struct instance (WORKS)
run_the_runnable(&MyRunnable, &x);
- // from closure (DOES NOT WORK)
- run_the_runnable(&|x| {
+ // from closure with annotated type (WORKS)
+ run_the_runnable(&|x: &i32| {
println!("Hello from run(&i32) closure {}", x);
}, &x);
}