不匹配的类型需要 `i8`，发现 `()`

Question

I am trying to make a temperature converter in Rust, but for some reason my code here gives me an error. I am trying to convert the choice String to an i8 type.

use std::io;

fn main() {
    println!("Welcome! Would you like to convert from (1) Fahrenheit to Celsius, or (2) from Celsius to Fahrenheit?");
    let mut choice = String::new();

    io::stdin()
        .read_line(&mut choice)
        .expect("Failed to read line");

    println!("{}", choice);

    let choice: i8 = match choice.trim().parse() {
        Ok(num) => num,
        Err(_) => {
            println!("Invalid input, please input a number.");
        }
    };
}

fn convert_to_c(f_temp: f64) {
    let mut c_temp: f64;
    c_temp = f_temp - 32.0 * 0.5556;
    println!("{}F in Celsius is: {}", f_temp, c_temp);
}

fn convert_to_f(c_temp: f64) {
    let mut f_temp: f64;
    f_temp = c_temp * 1.8 + 32.0;
    println!("{}C in Fahrenheit is: {}", c_temp, f_temp);
}

error[E0308]: mismatched types
  --> src/main.rs:15:19
   |
15 |           Err(_) => {
   |  ___________________^
16 | |             println!("Invalid input, please input a number.");
17 | |         }
   | |_________^ expected `i8`, found `()`

Answer 1

The problem is that this match statement is an expression that has to return an i8 to assign to choice , but the Err arm doesn't return an i8 .

    let choice: i8 = match choice.trim().parse() {
        Ok(num) => num,
        Err(_) => {
            println!("Invalid input, please input a number.");
        }
    };  

One way to get it to run, is to have the Err branch panic so it never returns at all.

        Err(_) => {
            println!("Invalid input, please input a number.");
            panic!();
        }

Answer 2

Your match arms both need to return the same type. Since you want to request new input from the user, there's no need to handle the Err case here, just loop until a correct value is entered.

This snippet loops until the value input can be parsed without issues. In that case if let Some(v).. it breaks with the value parsed. If the value cannot be parsed, it just loops along and tries again.

let mut input = String::new();
let choice: u8 = loop {
    io::stdin()
        .read_line(&mut input)
        .expect("Failed to read line");

    println!("{}", input);

    if let Some(v) = input.trim().parse().ok() {
        break v;
    }
    input.clear();
    println!("Invalid input. Please input a number.");
};