Rust - Error Conversion
Rust does not have an exception mechanism for error handling they provided the following two types.
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enum Option<T> {
Some(T),
None
}
enum Result<T, E> {
Ok(T),
Err(E)
}
So often user needs to convert errors for using many libraries because each library will provide a different error type on handling errors.
Let’s have a look below example.
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enum LibError {
GeneralError,
UnknownErr
}
enum MyAppError {
Error(String)
}
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fn foo() -> Result<String, MyAppError> {
let lib_result = lib_func();
let app_result = match lib_result {
Ok(lib_res) => format!("final result: {}", lib_res),
Err(err) => {
match err {
LibError::GeneralError => return Err(MyAppError::Error("General error".to_string())),
LibError::UnknownErr => return Err(MyAppError::Error("Unknown error".to_string())),
}
}
}
Ok(app_result)
}
Above code is very annoying to have match clause for every error case so we can use the From<T> trait to simplify it.
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trait From<T> {
fn from(value: T) -> Self;
}
impl From<LibError> for MyAppError {
fn from(value: LibError) -> Self {
match value {
LibError::GeneralError => return Err(MyAppError::Error("General error".to_string())),
LibError::UnknownErr => return Err(MyAppError::Error("Unknown error".to_string())),
}
}
}
Then we can use the following code for error.
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fn foo() -> Result<String, MyAppError> {
let lib_result = lib_func()?;
Ok(format!("final result: {}", lib_res))
}
Actually, complier will expand the above code like this.
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fn foo() -> Result<String, MyAppError> {
let lib_result = lib_func() {
Ok(lib_result) => lib_result,
Err(error) => return Err(From::from(error)),
}
Ok(format!("final result: {}", lib_res))
}
Defining application specific error enum type is another hustle so there are some libraries to remove duplicated work.
thiserror is for library project. anyhow or eyre for application project.
thiserror example on defining a custom error type.
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use thiserror::Error;
#[derive(Debug, Error)]
pub enum Error {
#[error(transparent)]
ImageError(image::error::ImageError),
#[error("not valid base64 image string")]
NotValidBase64,
#[error("failed to convert an image")]
ImageConvertFailure,
#[error("out of index on finding gaussian")]
GaussianIndexError,
#[error("failed to create a color quantization")]
FailureColorQuantization,
#[error("maximum supported number of quantization color is 256")]
NotSupportedNumberOfColorForQuantization,
#[error("failed to generate layers")]
LayerGenerationFailure,
#[error("failed to generate scan paths")]
ScanPathGenerationFailure,
#[error("failed to generate batch interpolation list")]
BatchInterpolationGenerationFailure,
#[error("failed image path tracing")]
ImagePathTracingFailure,
#[error("failed to generate svg string")]
FailureGeneratingSvgString,
#[error("unknown error")]
Unknown,
}