如何声明闭包参数的生存期? [英] How to declare a lifetime for a closure argument?
问题描述
我想为Rust中的闭包声明生存期,但是我找不到添加生存期声明的方法.
I would like to declare a lifetime for a closure in Rust, but I can't find a way to add a lifetime declaration.
use std::str::SplitWhitespace;
pub struct ParserError {
pub message: String,
}
fn missing_token(line_no: usize) -> ParserError {
ParserError {
message: format!("Missing token on line {}", line_no),
}
}
fn process_string(line: &str, line_number: usize) -> Result<(), ParserError> {
let mut tokens = line.split_whitespace();
match try!(tokens.next().ok_or(missing_token(line_number))) {
"hi" => println!("hi"),
_ => println!("Something else"),
}
// The following code gives "cannot infer appropriate lifetime.....
// let nt = |t: &mut SplitWhitespace| t.next().ok_or(missing_token(line_number));
// match try!(nt(&mut tokens)) {
// "there" => println!("there"),
// _ => println!("_"),
// }
// Where should I declare the lifetime 'a?
// let nt = |t: &'a mut SplitWhitespace| t.next().ok_or(missing_token(line_number));
// match try!(nt(&mut tokens)) {
// "there" => println!("there"),
// _ => println!("_"),
// }
return Ok(());
}
fn main() {
process_string("Hi there", 5).ok().expect("Error!!!");
process_string("", 5).ok().expect("Error!!! 2");
}
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
--> src/main.rs:22:42
|
22 | let nt = |t: &mut SplitWhitespace| t.next().ok_or(missing_token(line_number));
| ^^^^
|
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 22:14...
--> src/main.rs:22:14
|
22 | let nt = |t: &mut SplitWhitespace| t.next().ok_or(missing_token(line_number));
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: ...so that the types are compatible:
expected std::iter::Iterator
found std::iter::Iterator
note: but, the lifetime must be valid for the call at 23:16...
--> src/main.rs:23:16
|
23 | match try!(nt(&mut tokens)) {
| ^^^^^^^^^^^^^^^
note: ...so type `std::result::Result<&str, ParserError>` of expression is valid during the expression
--> src/main.rs:23:16
|
23 | match try!(nt(&mut tokens)) {
| ^^^^^^^^^^^^^^^
如何为该关闭声明生存期'a
?
How can I declare the lifetime 'a
for this closure?
推荐答案
&mut SplitWhitespace
实际上是&'b mut SplitWhitespace<'a>
.此处的相关生存期为'a
,因为它指定了next
返回的字符串片段的生存期.由于在line
参数上应用了split_whitespace
函数,因此需要将'a
设置为与line
参数相同的生存期.
The &mut SplitWhitespace
is actually a &'b mut SplitWhitespace<'a>
. The relevant lifetime here is the 'a
, as it specifies how long the string slices that next
returns live. Since you applied the split_whitespace
function on your line
argument, you need to set 'a
to the same lifetime that the line
argument has.
因此,第一步是将生存期添加到line
:
So as a first step you add a lifetime to line
:
fn process_string<'a>(line: &'a str, line_number: usize) -> Result<(), ParserError> {
,然后将生存期添加到闭包中的类型中:
and then you add the lifetime to the type in your closure:
let nt = |t: &mut SplitWhitespace<'a>| t.next().ok_or(missing_token(line_number));
请注意,虽然这可以回答您的问题,但正确的解决方案是 @AB的解决方案.
Note that while this answers your question, the correct solution to your Problem is @A.B.'s solution.
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