基本上,我想编写一个返回闭包的函数。我如何做到这一点而不必返回Box<FnOnce(u32)>
?
从closures chapter of the rust book中,我读到闭包只是structt和FnOnce
的隐式语法糖。这是我的尝试:
#[derive(Debug)]
struct MyError {
code: u32,
location: &'static str,
}
// Here is my closure:
struct MyErrorPartial {
location: &'static str,
}
impl FnOnce(u32) for MyErrorPartial {
type Output = MyError;
fn call_once(self, args: u32) -> MyError {
MyError {
code: args,
location: self.location,
}
}
}
fn error_at(location: &'static str) -> MyErrorPartial {
MyErrorPartial {location: location}
}
fn function_returning_code() -> Result<(), u32> {
Err(123)
}
fn function_with_error() -> Result<(), MyError> {
try!(function_returning_code().map_err(error_at("line1")));
try!(function_returning_code().map_err(error_at("line2")));
Ok(())
}
fn main() {
function_with_error().unwrap();
}
当前给出一个错误:
<anon>:11:12: 11:17 error: associated type bindings are not allowed here [E0229]
<anon>:11 impl FnOnce(u32) for MyErrorPartial {
^~~~~
最佳答案
在结构上手动实现Fn*
特征的语法是这样的:
impl FnOnce<(Arg1,Arg2,Arg3,)> for MyStruct {
type Output = MyOutput;
extern "rust-call" fn call_once(args: (Arg1, Arg2, Arg3,)) -> MyOutput {
// implementation here
}
}
请注意,所有参数都作为一个元组给出。
此外,此语法不稳定,需要
#![feature(core, unboxed_closures)]
,因此您只能在夜间使用,不能在beta channel 上使用它。在您的情况下,它会这样翻译:
impl FnOnce<(u32,)> for MyErrorPartial {
type Output = MyError;
extern "rust-call" fn call_once(self, args: (u32,)) -> MyError {
MyError {
code: args.0,
location: self.location,
}
}
}