我如何返回一个由带有 &'a mut self 的函数生成的迭代器(当 self 在本地创建时)? [英] How do I return an Iterator that's generated by a function that takes &'a mut self (when self is created locally)?
问题描述
更新:帖子标题已更新，答案已移出问题.简短的回答是你不能.请参阅我对这个问题的回答.
我正在关注错误处理博客文章这里(github 是此处)，我尝试对代码进行一些修改，以便search 函数返回一个 Iterator 而不是 Vec.这太难了，我被卡住了.
I'm following an Error Handling blog post here (github for it is here), and I tried to make some modifications to the code so that the search function returns an Iterator instead of a Vec. This has been insanely difficult, and I'm stuck.
我已经到了这一点:
fn search<'a, P: AsRef<Path>>(file_path: &Option<P>, city: &str)
-> Result<FilterMap<csv::reader::DecodedRecords<'a, Box<Read>, Row>,
FnMut(Result<Row, csv::Error>)
-> Option<Result<PopulationCount, csv::Error>>>,
CliError> {
let mut found = vec![];
let input: Box<io::Read> = match *file_path {
None => Box::new(io::stdin()),
Some(ref file_path) => Box::new(try!(fs::File::open(file_path))),
};
let mut rdr = csv::Reader::from_reader(input);
let closure = |row: Result<Row, csv::Error>| -> Option<Result<PopulationCount, csv::Error>> {
let row = match row {
Ok(row) => row,
Err(err) => return Some(Err(From::from(err))),
};
match row.population {
None => None,
Some(count) => if row.city == city {
Some(Ok(PopulationCount {
city: row.city,
country: row.country,
count: count,
}))
} else {
None
}
}
};
let found = rdr.decode::<Row>().filter_map(closure);
if !found.all(|row| match row {
Ok(_) => true,
_ => false,
}) {
Err(CliError::NotFound)
} else {
Ok(found)
}
}
编译器出现以下错误:
src/main.rs:97:1: 133:2 error: the trait `core::marker::Sized` is not implemented for the type `core::ops::FnMut(core::result::Result<Row, csv::Error>) -> core::option::Option<core::result::Result<PopulationCount, csv::Error>>` [E0277]
src/main.rs:97 fn search<'a, P: AsRef<Path>>(file_path: &Option<P>, city: &str) -> Result<FilterMap<csv::reader::DecodedRecords<'a, Box<Read>, Row>, FnMut(Result<Row, csv::Error>) -> Option<Result<PopulationCount, csv::Error>>>, CliError> {
src/main.rs:98 let mut found = vec![];
src/main.rs:99 let input: Box<io::Read> = match *file_path {
src/main.rs:100 None => Box::new(io::stdin()),
src/main.rs:101 Some(ref file_path) => Box::new(try!(fs::File::open(file_path))),
src/main.rs:102 };
...
src/main.rs:97:1: 133:2 note: `core::ops::FnMut(core::result::Result<Row, csv::Error>) -> core::option::Option<core::result::Result<PopulationCount, csv::Error>>` does not have a constant size known at compile-time
src/main.rs:97 fn search<'a, P: AsRef<Path>>(file_path: &Option<P>, city: &str) -> Result<FilterMap<csv::reader::DecodedRecords<'a, Box<Read>, Row>, FnMut(Result<Row, csv::Error>) -> Option<Result<PopulationCount, csv::Error>>>, CliError> {
src/main.rs:98 let mut found = vec![];
src/main.rs:99 let input: Box<io::Read> = match *file_path {
src/main.rs:100 None => Box::new(io::stdin()),
src/main.rs:101 Some(ref file_path) => Box::new(try!(fs::File::open(file_path))),
src/main.rs:102 };
...
error: aborting due to previous error
我也试过这个函数定义:
I've also tried this function definition:
fn search<'a, P: AsRef<Path>, F>(file_path: &Option<P>, city: &str)
-> Result<FilterMap<csv::reader::DecodedRecords<'a, Box<Read>, Row>, F>,
CliError>
where F: FnMut(Result<Row, csv::Error>)
-> Option<Result<PopulationCount, csv::Error>> {
来自编译器的这些错误:
with these errors from the compiler:
src/main.rs:131:12: 131:17 error: mismatched types:
expected `core::iter::FilterMap<csv::reader::DecodedRecords<'_, Box<std::io::Read>, Row>, F>`,
found `core::iter::FilterMap<csv::reader::DecodedRecords<'_, Box<std::io::Read>, Row>, [closure src/main.rs:105:19: 122:6]>`
(expected type parameter,
found closure) [E0308]
src/main.rs:131 Ok(found)
我不能Box闭包，因为这样它就不会被filter_map接受.
I can't Box the closure because then it won't be accepted by filter_map.
然后我尝试了这个:
fn search<'a, P: AsRef<Path>>(file_path: &Option<P>, city: &'a str)
-> Result<(Box<Iterator<Item=Result<PopulationCount, csv::Error>> + 'a>, csv::Reader<Box<io::Read>>), CliError> {
let input: Box<io::Read> = match *file_path {
None => box io::stdin(),
Some(ref file_path) => box try!(fs::File::open(file_path)),
};
let mut rdr = csv::Reader::from_reader(input);
let mut found = rdr.decode::<Row>().filter_map(move |row| {
let row = match row {
Ok(row) => row,
Err(err) => return Some(Err(err)),
};
match row.population {
None => None,
Some(count) if row.city == city => {
Some(Ok(PopulationCount {
city: row.city,
country: row.country,
count: count,
}))
},
_ => None,
}
});
if found.size_hint().0 == 0 {
Err(CliError::NotFound)
} else {
Ok((box found, rdr))
}
}
fn main() {
let args: Args = Docopt::new(USAGE)
.and_then(|d| d.decode())
.unwrap_or_else(|err| err.exit());
match search(&args.arg_data_path, &args.arg_city) {
Err(CliError::NotFound) if args.flag_quiet => process::exit(1),
Err(err) => fatal!("{}", err),
Ok((pops, rdr)) => for pop in pops {
match pop {
Err(err) => panic!(err),
Ok(pop) => println!("{}, {}: {} - {:?}", pop.city, pop.country, pop.count, rdr.byte_offset()),
}
}
}
}
这给了我这个错误:
src/main.rs:107:21: 107:24 error: `rdr` does not live long enough
src/main.rs:107 let mut found = rdr.decode::<Row>().filter_map(move |row| {
^~~
src/main.rs:100:117: 130:2 note: reference must be valid for the lifetime 'a as defined on the block at 100:116...
src/main.rs:100 -> Result<(Box<Iterator<Item=Result<PopulationCount, csv::Error>> + 'a>, csv::Reader<Box<io::Read>>), CliError> {
src/main.rs:101 let input: Box<io::Read> = match *file_path {
src/main.rs:102 None => box io::stdin(),
src/main.rs:103 Some(ref file_path) => box try!(fs::File::open(file_path)),
src/main.rs:104 };
src/main.rs:105
...
src/main.rs:106:51: 130:2 note: ...but borrowed value is only valid for the block suffix following statement 1 at 106:50
src/main.rs:106 let mut rdr = csv::Reader::from_reader(input);
src/main.rs:107 let mut found = rdr.decode::<Row>().filter_map(move |row| {
src/main.rs:108 let row = match row {
src/main.rs:109 Ok(row) => row,
src/main.rs:110 Err(err) => return Some(Err(err)),
src/main.rs:111 };
...
error: aborting due to previous error
是我设计错了，还是我采取了错误的方法?我错过了一些非常简单和愚蠢的东西吗?我不知道从这里去哪里.
Have I designed something wrong, or am I taking the wrong approach? Am I missing something really simple and stupid? I'm not sure where to go from here.
推荐答案
此答案基于 @bluss 的answer + irc.mozilla.org 上 #rust 的帮助
This answer is based on @bluss's answer + help from #rust on irc.mozilla.org
从代码中不明显的一个问题，并导致上面显示的最终错误，与 csv::Reader::decode(参见其来源).它需要 &'a mut self，这个问题的解释包含在这个 answer.这基本上会导致读取器的生命周期受限于它被调用的块.解决这个问题的方法是将函数分成两半(因为我无法控制函数定义，如上一个答案链接中所建议的那样).我需要一个在 main 函数中有效的阅读器生命周期，这样阅读器就可以被传递到 search 函数中.请参阅下面的代码(肯定可以清理更多):
One issue that's not obvious from the code, and which was causing the final error displayed just above, has to do with the definition of csv::Reader::decode (see its source). It takes &'a mut self, the explanation of this problem is covered in this answer. This essentially causes the lifetime of the reader to be bounded to the block it's called in. The way to fix this is to split the function in half (since I can't control the function definition, as recommended in the previous answer link). I needed a lifetime on the reader that was valid within the main function, so the reader could then be passed down into the search function. See the code below (It could definitely be cleaned up more):
fn population_count<'a, I>(iter: I, city: &'a str)
-> Box<Iterator<Item=Result<PopulationCount,csv::Error>> + 'a>
where I: IntoIterator<Item=Result<Row,csv::Error>>,
I::IntoIter: 'a,
{
Box::new(iter.into_iter().filter_map(move |row| {
let row = match row {
Ok(row) => row,
Err(err) => return Some(Err(err)),
};
match row.population {
None => None,
Some(count) if row.city == city => {
Some(Ok(PopulationCount {
city: row.city,
country: row.country,
count: count,
}))
},
_ => None,
}
}))
}
fn get_reader<P: AsRef<Path>>(file_path: &Option<P>)
-> Result<csv::Reader<Box<io::Read>>, CliError>
{
let input: Box<io::Read> = match *file_path {
None => Box::new(io::stdin()),
Some(ref file_path) => Box::new(try!(fs::File::open(file_path))),
};
Ok(csv::Reader::from_reader(input))
}
fn search<'a>(reader: &'a mut csv::Reader<Box<io::Read>>, city: &'a str)
-> Box<Iterator<Item=Result<PopulationCount, csv::Error>> + 'a>
{
population_count(reader.decode::<Row>(), city)
}
fn main() {
let args: Args = Docopt::new(USAGE)
.and_then(|d| d.decode())
.unwrap_or_else(|err| err.exit());
let reader = get_reader(&args.arg_data_path);
let mut reader = match reader {
Err(err) => fatal!("{}", err),
Ok(reader) => reader,
};
let populations = search(&mut reader, &args.arg_city);
let mut found = false;
for pop in populations {
found = true;
match pop {
Err(err) => fatal!("fatal !! {}", err),
Ok(pop) => println!("{}, {}: {}", pop.city, pop.country, pop.count),
}
}
if !(found || args.flag_quiet) {
fatal!("{}", CliError::NotFound);
}
}
我在尝试使其工作时学到了很多东西，并且对编译器错误有了更多的了解.现在很明显，如果这是 C，上面的最后一个错误实际上可能会导致段错误，这将更难调试.我还意识到，从预先计算的 vec 转换为迭代器需要更多地考虑内存何时进入和超出范围；我不能仅仅改变几个函数调用和返回类型就可以了.
I've learned a lot trying to get this to work, and have much more appreciation for the compiler errors. It's now clear that had this been C, the last error above could actually have caused segfaults, which would have been much harder to debug. I've also realized that converting from a pre-computed vec to an iterator requires more involved thinking about when the memory comes in and out of scope; I can't just change a few function calls and return types and call it a day.
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