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

Question

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.

Rubens Brandão · Accepted Answer · 2022-07-03 22:20:51Z

1

If you want to have a different Error for each implementation, you can define the trait with an associated error type, just like in TryFrom:

pub trait Decoder {
    type Error;
    fn do_something(&self) -> Result<(), Self::Error>;
}

pub enum SpecificDecoder1Error {}
pub struct SpecificDecoder1 {}
impl Decoder for SpecificDecoder1 {
    type Error = SpecificDecoder1Error;
    fn do_something(&self) -> Result<(), Self::Error> {
        todo!()
    }
}

pub enum SpecificDecoder2Error {}
pub struct SpecificDecoder2 {}
impl Decoder for SpecificDecoder2 {
    type Error = SpecificDecoder2Error;
    fn do_something(&self) -> Result<(), Self::Error> {
        todo!()
    }
}

But if you want the trait to return a generic type, you can return a dyn std::any::Any value, and downcast if necessary:

pub trait Decoder {
    fn do_something(&self) -> Result<(), Box<dyn std::any::Any>>;
}

pub struct SpecificDecoder1Error(u16);
pub struct SpecificDecoder1 {}
impl Decoder for SpecificDecoder1 {
    fn do_something(&self) -> Result<(), Box<dyn std::any::Any>> {
        Err(Box::new(SpecificDecoder1Error(13)))
    }
}

pub struct SpecificDecoder2Error(String);
pub struct SpecificDecoder2 {}
impl Decoder for SpecificDecoder2 {
    fn do_something(&self) -> Result<(), Box<dyn std::any::Any>> {
        Err(Box::new(SpecificDecoder2Error(
            "Specific Error 2".to_string(),
        )))
    }
}

fn main() {
    let dec1 = SpecificDecoder1 {};
    let dec2 = SpecificDecoder2 {};
    let decs: [&dyn Decoder; 2] = [&dec1, &dec2];
    for dec in decs {
        let error = dec.do_something().err().unwrap();
        if let Some(error1) = error.downcast_ref::<SpecificDecoder1Error>() {
            println!("Error from SpecificDecoder1Error: {}", error1.0)
        } else if let Some(error2) =
            error.downcast_ref::<SpecificDecoder2Error>()
        {
            println!("Error from SpecificDecoder2Error: {}", error2.0)
        } else {
            unreachable!()
        }
    }
}

answered Jul 3, 2022 at 22:20

Rubens Brandão

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2 Comments

Poperton Over a year ago

my concern was about using ? on the dyn Any. Creating error boxes all the time out be bad. Do you think there's a nice way of not needing to create a box every time?

Rubens Brandão Over a year ago

Yes, avoiding using box when unnecessary is a good idea. But it depends a lot about the citation. Usually errors are returned only once, because lots of times when they happen, the program abort it's execution/task, so maybe in you case is not a big deal.

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