Generic: expected type parameter `E`, found enum `MyError`

help
1

Hello,
I'm currently trying to create a custom std::error::Error enum.
But when MyError is used in Generic, I get the following compilation error:

use std::{error::Error, fmt};

#[derive(Debug)]
enum MyError {
    Hello
}

impl fmt::Display for MyError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        panic!("hello");
    }
}

impl Error for MyError {
    fn description(&self) -> &str {
        panic!("hello");
    }
}

fn test<E>()-> Result<(), E>
    where
        E: Error
{
    Err(MyError::Hello)
}

24 |     Err(MyError::Hello)
   |     --- ^^^^^^^^^^^^^^ expected type parameter `E`, found enum `MyError`
   |     |
   |     arguments to this enum variant are incorrect
   |
   = note: expected type parameter `E`
                        found enum `MyError`

I have started to learn rust recently. I would be grateful if you could help me on this error. I've seen a lot of similar issues but for me it's difficult to relate to my problem.

playground

2

Type parameters are chosen by the caller; what should happen if someone were to call it with test::<NotMyError>()? It wouldn't make sense. You should just return Result<(), MyError> instead.

1 Like
3

Thank you very much. It's clear now. :wink: