typederror

A wrapper around anyhow but with a "primary" error type.

Motivation

This library aims to be the glue between anyhow and thiserror. It allows you to define a primary error type for variants that the caller should match on, while still capturing any other errors that may have occurred along the way.

Documenting the error type of a function

If you simply return an anyhow::Error, the caller has no idea what kind of error to expect. They would need to read your code to determine what the possible error types are.

By using TError, you can specify the primary error type that the caller should match on. This has the effect of documenting the primary error type for your function.

fn my_fallible_function() -> typederror::Result<(), MyError> {
    // Do something that might fail.
    let s = std::fs::read_to_string("file.txt").map_err(|e| MyError::IoError(e))?;
    // NOTE: if `MyError` implements `From<std::io::Error>`,
    // you can do `std::fs::read_to_string("file.txt").terror()?` instead.
    some_operation(s)?; // An error we don't need to match on.
    Ok(())
}

The primary error type could be an enum that derives thiserror::Error, where only the meaningful errors are captured by the enum and any other errors are captured by the anyhow::Error underneath.

You can also implement DefaultError so that all other errors are captured in a special "catch-all" variant of the primary error type.

#[derive(Debug, thiserror::Error)]
enum MyError {
   #[error("IO error: {0}")]
   IoError(#[from] std::io::Error),
   #[error("{0}")]
   Misc(typederror::anyhow::Error)
}

impl DefaultError for MyError {
    fn from_anyhow(err: typederror::anyhow::Error) -> Self {
        Self::Misc(err)
    }
}

Downcasting to the primary error type

Since TError already knows the primary error type, it can provide convenience methods for downcasting to that type. This allows you to more easily work with errors of a single type without needing to match on several different error types.

if let Err(err) = my_fallible_function() { // returns Result<T, TError<MyError>>
    match err.get() {
        MyError::IoError(e) => { // e is of type `std::io::Error`
            // Handle the error.
        }
        MyError::Misc(e) => { // e is of type `anyhow::Error`
            // Handle the error.
        }
    }
}

You can also downcast to other types if needed, the same as you would with anyhow.

match err.downcast_ref::<serde::Error>() {
    Ok(e) => {
        // Handle serde error.
    }
    Err(e) => {
        // Handle other error.
    }
}

Start simple and add error variants later

To get you started, you can use TError<()> as the primary error type. Or use typederror::Result<T> as the return type of your function. This will effectively work the same as anyhow, allowing you to write your code and worry about error types later.

fn do_something() -> typederror::Result<()> {
    // Do something.
    my_fallible_function()?;
    Ok(())
}

Later, when you want to create specific variants for your function for easier matching by the caller, you can create an enum, derive thiserror::Error, and use that as the primary error type instead. You will need to add any necessary conversions, but you only need to add the variants you want to match on.

All other errors will still be captured as per anyhow behaviour, or they can be captured in a special "catch-all" variant of your enum by implementing the DefaultError trait on the enum.

Caveats

Unfortunately the ? operator cannot automatically convert error types to your primary error type.

For example:

#[derive(Debug, thiserror::Error)]
enum MyError {
    #[error("IO error: {0}")]
    IoError(#[from] std::io::Error),
    #[error("{0}")]
    Misc(anyhow::Error)
}

impl DefaultError for MyError {
    fn from_anyhow(err: anyhow::Error) -> Self {
        Self::Misc(err)
    }
}

fn my_fallible_function() -> typederror::Result<(), MyError> {
   let s = std::fs::read_to_string("file.txt")?;
   // Do something else with s.
   Ok(())
}

fn main() {
    if let Err(e) = my_fallible_function() {
        match e.get() {
            // ...
        }
    }
}

In the above example, the ? operator will not automatically convert the std::io::Error to MyError::IoError, as it would if you had used MyError as the error type directly. The error would instead match as MyError::Misc in the call to e.get().

To capture the IoError correctly, change the first line of the function to

let s = std::fs::read_to_string("file.txt").terror()?;

Minimum Supported Rust Version

The MSRV for thirtyfour is currently 1.75 and will be updated as needed by dependencies.

License