如何在结构上创建一个大小的闭包或实现Fn/FnMut/FnOnce？

Question

基本上,我想编写一个返回闭包的函数.我怎么能这样做而不必返回Box<FnOnce(u32)>？

从Rust书的闭包章节,我读到一个闭包只是一个结构的语法糖和一个impl FnOnce.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();
}

它目前给出一个错误:

<anon>:11:12: 11:17 error: associated type bindings are not allowed here [E0229]
<anon>:11 impl FnOnce(u32) for MyErrorPartial {
                     ^~~~~

Answer 1

Fn*在结构上手动实现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,所有参数都以单个元组的形式给出.

此外,此语法不稳定且需要#![feature(core, unboxed_closures)],因此您无法在测试版频道上使用它,只能在夜间播放.

在你的情况下,它会像这样翻译:

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,
        }
    }
}