将嵌套的for循环转换为迭代器

Question

Rust--新手在这里，我在将以下嵌套的for循环转换为迭代器时遇到了问题：

#![allow(unused)]

use std::io::Error;

fn main() {
    let realms = vec!["realm".to_string()];
    let auctions = get_auctions(&realms);
}

pub struct Auction;
pub struct RealmAuctionFile;

fn get_realm_auctions(file: &RealmAuctionFile) -> Result<Vec<Auction>, Error> {
    let auctions = vec![Auction {}];

    Ok(auctions)
}

fn get_realm_auction_files(realm: &str) -> Result<Vec<RealmAuctionFile>, Error> {
    let files = vec![RealmAuctionFile {}];

    Ok(files)
}

pub fn get_auctions(realms: &Vec<String>) -> Result<Vec<Auction>, Error> {
    let mut auctions = vec![];

    for realm in realms.iter() {
        let files = get_realm_auction_files(realm)?;

        for file in files.iter() {
            let mut realm_auctions = get_realm_auctions(file)?;

            auctions.append(&mut realm_auctions);
        }
    }

    Ok(auctions)
}

pub fn get_auctions_with_iterator(realms: &Vec<String>) -> Result<Vec<Auction>, Error> {
    let auctions = realms
        .iter()
        .flat_map(|realm| {
            let realms_auctions: Vec<Auction> = get_realm_auction_files(realm)?
                .iter()
                .flat_map(|file| {
                    let auctions = get_realm_auctions(file)?;

                    Ok(auctions)
                })
                .collect();

            Ok(realms_auctions)
        })
        .collect();

    Ok(auctions)
}

Playground

我得到两个错误：

error[E0277]: a collection of type `std::vec::Vec<Auction>` cannot be built from an iterator over elements of type `std::vec::Vec<Auction>`
  --> src/main.rs:52:18
   |
52 |                 .collect();
   |                  ^^^^^^^ a collection of type `std::vec::Vec<Auction>` cannot be built from `std::iter::Iterator<Item=std::vec::Vec<Auction>>`
   |
   = help: the trait `std::iter::FromIterator<std::vec::Vec<Auction>>` is not implemented for `std::vec::Vec<Auction>`

error[E0277]: a collection of type `std::vec::Vec<Auction>` cannot be built from an iterator over elements of type `std::vec::Vec<Auction>`
  --> src/main.rs:56:10
   |
56 |         .collect();
   |          ^^^^^^^ a collection of type `std::vec::Vec<Auction>` cannot be built from `std::iter::Iterator<Item=std::vec::Vec<Auction>>`
   |
   = help: the trait `std::iter::FromIterator<std::vec::Vec<Auction>>` is not implemented for `std::vec::Vec<Auction>`

而且，我不能用更常用的?来“转换”.unwrap()

Answer 1

不要展开。而是生成Result的迭代器，并利用Result implements FromIterator将其收集到Result<Vec>中的事实。

另请参见this answer。