generics - 具有特征的通用函数，用于读取提示错误特征的数字

Question

我正在尝试创建一个通用函数来从标准输入读取数字:

use std::error::Error;
use std::io;
use std::io::Write;
use std::str::FromStr;

fn read_number<F: FromStr>(prompt: &str) -> Result<F, Box<Error>> {
    let mut guess = String::new();
    let mut sout = io::stdout();

    sout.write(prompt.as_bytes())?;

    sout.flush()?;

    io::stdin().read_line(&mut guess)?;

    Ok(guess.trim().parse()?)
}

fn read_until_number<F: FromStr>(prompt: &str) -> F {
    loop {
        match read_number(prompt) {
            Ok(num) => break num,
            Err(_) => println!("Please enter valid number."),
        };
    }
}

fn main() {
    let x: u32 = read_until_number("Enter integer:\n");
    let y: f32 = read_until_number("Enter float:\n");
    println!("You entered this integer: {} and this float: {}", x, y);
}

它不起作用；我收到以下错误:

error[E0277]: the trait bound `<F as std::str::FromStr>::Err: std::error::Error` is not satisfied
  --> src/main.rs:16:8
   |
16 |     Ok(guess.trim().parse()?)
   |        ^^^^^^^^^^^^^^^^^^^^^ the trait `std::error::Error` is not implemented for `<F as std::str::FromStr>::Err`
   |
   = help: consider adding a `where <F as std::str::FromStr>::Err: std::error::Error` bound
   = note: required because of the requirements on the impl of `std::convert::From<<F as std::str::FromStr>::Err>` for `std::boxed::Box<std::error::Error>`
   = note: required by `std::convert::From::from`

Answer 1

错误信息告诉你该怎么做:

consider adding a `where <F as std::str::FromStr>::Err: std::error::Error` bound

按照建议，但使用更简单的语法:

fn read_number<F>(prompt: &str) -> Result<F, Box<Error>>
where
    F: FromStr,
    F::Err: std::error::Error,

这会导致另一个错误，它也会告诉您该怎么做:

error[E0310]: the associated type `<F as std::str::FromStr>::Err` may not live long enough
  --> src/main.rs:20:8
   |
20 |     Ok(guess.trim().parse()?)
   |        ^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider adding an explicit lifetime bound `<F as std::str::FromStr>::Err: 'static`...
note: ...so that the type `<F as std::str::FromStr>::Err` will meet its required lifetime bounds
  --> src/main.rs:20:8
   |
20 |     Ok(guess.trim().parse()?)
   |        ^^^^^^^^^^^^^^^^^^^^^

遵循建议并将其添加到我们现有的限制中:

fn read_number<F>(prompt: &str) -> Result<F, Box<Error>>
where
    F: FromStr,
    F::Err: std::error::Error + 'static,

然后您将得到与 read_until_number 相同的错误功能。重复相同的过程，您最终得到:

use std::error::Error;
use std::io;
use std::io::Write;
use std::str::FromStr;

fn read_number<F>(prompt: &str) -> Result<F, Box<Error>>
where
    F: FromStr,
    F::Err: std::error::Error + 'static,
{
    let mut guess = String::new();
    let mut sout = io::stdout();

    sout.write(prompt.as_bytes())?;

    sout.flush()?;

    io::stdin().read_line(&mut guess)?;

    Ok(guess.trim().parse()?)
}

fn read_until_number<F>(prompt: &str) -> F
where
    F: FromStr,
    F::Err: std::error::Error + 'static,
{
    loop {
        match read_number(prompt) {
            Ok(num) => break num,
            Err(_) => println!("Please enter valid number."),
        };
    }
}

fn main() {
    let x: u32 = read_until_number("Enter integer:\n");
    let y: f32 = read_until_number("Enter float:\n");
    println!("You entered this integer: {} and this float: {}", x, y);
}

---

为什么需要这个？

1. implementation of From for Box<Error> 要求类型实现 std::error::Error ，但特征 FromStr 对关联的 Err 没有限制类型。您必须向您的函数添加限制才能执行转换。

2. 默认情况下，参数位置中的特征对象具有隐式 'static绑定(bind)，就像你做了一样Box<Error + 'static> .你可以改变read_number使用更细微的生命周期，但您无法更改 read_until_number因为错误必须存在于函数之外:

   fn read_number<'a, F>(prompt: &'a str) -> Result<F, Box<Error + 'a>>
   where
       F: FromStr,
       F::Err: std::error::Error + 'a,