Trait for something generic, but errors could be specific for trait implementation

help
1

Let's say I have a trait for a Decoder, and I have specific decoders that implement this trait:

pub trait Decoder {
    fn do_something(&self) -> Result<(), DoSomethingError>;
}

pub enum DoSomethingError {}

pub struct SpecificDecoder1 {}

impl Decoder for SpecificDecoder1 {
    fn do_something(&self) -> Result<(), DoSomethingError> {
        todo!()
    }
}

pub struct SpecificDecoder2 {}

impl Decoder for SpecificDecoder2 {
    fn do_something(&self) -> Result<(), DoSomethingError> {
        todo!()
    }
}

each decoder could return specific errors. However, I don't want to include these specific errors in the DoSomethingError trait as some decoders could not even be available on some devices, so I don't want the mess of #cfg(feature="...") on the enum.

I thought about

pub trait DoSomethingError {}
fn do_something(&self) -> Result<(), Box<dyn DoSomethingError>> {
    Ok(self.do_another_something()?)
}

this way I can downcast the Box<dyn DoSomethingError> to a specific error if needed. However, Ok(self.do_another_something()?) wouldn't work. Remember that I don't want to store a string, I want to be able to match the error somehow, or try to match it, so I cannot create a UserDefined(String) variant on the string and convert every error to it.

2

If the returned error type varies by the implementor of the trait, then it should probably be an associated type instead of anything concrete.

1 Like
3

For example, see the associated type Error in

4

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