當謂詞返回Result時，如何過濾迭代器 <bool, _> ？

Question

我希望迭代迭代器，但我的謂詞有可能失敗。 當謂詞失敗時，我想使整個函數失敗。 在這個例子中，我想work返回maybe生成的Result ：

fn maybe(v: u32) -> Result<bool, u8> {
    match v % 3 {
        0 => Ok(true),
        1 => Ok(false),
        2 => Err(42),
    }
}

fn work() -> Result<Vec<u32>, u8> {
    [1, 2, 3, 4, 5].iter().filter(|&&x| maybe(x)).collect()
}

fn main() {
    println!("{:?}", work())
}

error[E0308]: mismatched types
  --> src/main.rs:10:45
   |
10 |         [1, 2, 3, 4, 5].iter().filter(|&&x| maybe(x)).collect()
   |                                             ^^^^^^^^ expected bool, found enum `std::result::Result`
   |
   = note: expected type `bool`
              found type `std::result::Result<bool, u8>`

error[E0277]: the trait bound `std::result::Result<std::vec::Vec<u32>, u8>: std::iter::FromIterator<&u32>` is not satisfied
  --> src/main.rs:10:55
   |
10 |         [1, 2, 3, 4, 5].iter().filter(|&&x| maybe(x)).collect()
   |                                                       ^^^^^^^ a collection of type `std::result::Result<std::vec::Vec<u32>, u8>` cannot be built from an iterator over elements of type `&u32`
   |
   = help: the trait `std::iter::FromIterator<&u32>` is not implemented for `std::result::Result<std::vec::Vec<u32>, u8>`

Answer 1

您可以將Result<bool, u8>轉換為Option<Result<u32, u8>> ，即將bool拉出到Option並將值放入其中，然后使用filter_map ：

fn maybe(v: u32) -> Result<bool, u8> {
    match v % 3 {
        0 => Ok(true),
        1 => Ok(false),
        _ => Err(42),
    }
}

fn work() -> Result<Vec<u32>, u8> {
    [1, 2, 3, 4, 5]
        .iter()
        .filter_map(|&x| match maybe(x) {
            Ok(true) => Some(Ok(x)),
            Ok(false) => None,
            Err(e) => Some(Err(e)),
        })
        .collect()
}

fn main() {
    println!("{:?}", work())
}

操場